diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index ef729a779f..082e0d247c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,5 +1,8 @@
 on: [push, pull_request]
 name: Continuous Integration
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
 
 jobs:
   inspect-code:
diff --git a/osu.Game.Rulesets.Catch/CatchRuleset.cs b/osu.Game.Rulesets.Catch/CatchRuleset.cs
index 321399c597..999b9f6e2c 100644
--- a/osu.Game.Rulesets.Catch/CatchRuleset.cs
+++ b/osu.Game.Rulesets.Catch/CatchRuleset.cs
@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using osu.Framework.Extensions.EnumExtensions;
@@ -33,7 +31,7 @@ namespace osu.Game.Rulesets.Catch
 {
     public class CatchRuleset : Ruleset, ILegacyRuleset
     {
-        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod> mods = null) => new DrawableCatchRuleset(this, beatmap, mods);
+        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod>? mods = null) => new DrawableCatchRuleset(this, beatmap, mods);
 
         public override ScoreProcessor CreateScoreProcessor() => new CatchScoreProcessor();
 
diff --git a/osu.Game.Rulesets.Mania/Configuration/ManiaRulesetConfigManager.cs b/osu.Game.Rulesets.Mania/Configuration/ManiaRulesetConfigManager.cs
index 64e1f75e2c..d367c82ed8 100644
--- a/osu.Game.Rulesets.Mania/Configuration/ManiaRulesetConfigManager.cs
+++ b/osu.Game.Rulesets.Mania/Configuration/ManiaRulesetConfigManager.cs
@@ -11,7 +11,7 @@ namespace osu.Game.Rulesets.Mania.Configuration
 {
     public class ManiaRulesetConfigManager : RulesetConfigManager<ManiaRulesetSetting>
     {
-        public ManiaRulesetConfigManager(SettingsStore settings, RulesetInfo ruleset, int? variant = null)
+        public ManiaRulesetConfigManager(SettingsStore? settings, RulesetInfo ruleset, int? variant = null)
             : base(settings, ruleset, variant)
         {
         }
diff --git a/osu.Game.Rulesets.Mania/ManiaRuleset.cs b/osu.Game.Rulesets.Mania/ManiaRuleset.cs
index 813e2c461a..c6a5e8bdaa 100644
--- a/osu.Game.Rulesets.Mania/ManiaRuleset.cs
+++ b/osu.Game.Rulesets.Mania/ManiaRuleset.cs
@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -48,7 +46,7 @@ namespace osu.Game.Rulesets.Mania
         /// </summary>
         public const int MAX_STAGE_KEYS = 10;
 
-        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod> mods = null) => new DrawableManiaRuleset(this, beatmap, mods);
+        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod>? mods = null) => new DrawableManiaRuleset(this, beatmap, mods);
 
         public override ScoreProcessor CreateScoreProcessor() => new ManiaScoreProcessor();
 
@@ -285,7 +283,7 @@ namespace osu.Game.Rulesets.Mania
 
         public override IConvertibleReplayFrame CreateConvertibleReplayFrame() => new ManiaReplayFrame();
 
-        public override IRulesetConfigManager CreateConfig(SettingsStore settings) => new ManiaRulesetConfigManager(settings, RulesetInfo);
+        public override IRulesetConfigManager CreateConfig(SettingsStore? settings) => new ManiaRulesetConfigManager(settings, RulesetInfo);
 
         public override RulesetSettingsSubsection CreateSettings() => new ManiaSettingsSubsection(this);
 
diff --git a/osu.Game.Rulesets.Osu/Edit/Blueprints/Sliders/Components/PathControlPointPiece.cs b/osu.Game.Rulesets.Osu/Edit/Blueprints/Sliders/Components/PathControlPointPiece.cs
index 92d83d900e..ab4b492767 100644
--- a/osu.Game.Rulesets.Osu/Edit/Blueprints/Sliders/Components/PathControlPointPiece.cs
+++ b/osu.Game.Rulesets.Osu/Edit/Blueprints/Sliders/Components/PathControlPointPiece.cs
@@ -6,6 +6,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using JetBrains.Annotations;
 using osu.Framework.Allocation;
 using osu.Framework.Bindables;
 using osu.Framework.Extensions.Color4Extensions;
@@ -53,6 +54,9 @@ namespace osu.Game.Rulesets.Osu.Edit.Blueprints.Sliders.Components
         private IBindable<Vector2> sliderPosition;
         private IBindable<float> sliderScale;
 
+        [UsedImplicitly]
+        private readonly IBindable<int> sliderVersion;
+
         public PathControlPointPiece(Slider slider, PathControlPoint controlPoint)
         {
             this.slider = slider;
@@ -61,7 +65,8 @@ namespace osu.Game.Rulesets.Osu.Edit.Blueprints.Sliders.Components
             // we don't want to run the path type update on construction as it may inadvertently change the slider.
             cachePoints(slider);
 
-            slider.Path.Version.BindValueChanged(_ =>
+            sliderVersion = slider.Path.Version.GetBoundCopy();
+            sliderVersion.BindValueChanged(_ =>
             {
                 cachePoints(slider);
                 updatePathType();
diff --git a/osu.Game.Rulesets.Osu/OsuRuleset.cs b/osu.Game.Rulesets.Osu/OsuRuleset.cs
index 226299d168..6af765361d 100644
--- a/osu.Game.Rulesets.Osu/OsuRuleset.cs
+++ b/osu.Game.Rulesets.Osu/OsuRuleset.cs
@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -44,7 +42,7 @@ namespace osu.Game.Rulesets.Osu
 {
     public class OsuRuleset : Ruleset, ILegacyRuleset
     {
-        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod> mods = null) => new DrawableOsuRuleset(this, beatmap, mods);
+        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod>? mods = null) => new DrawableOsuRuleset(this, beatmap, mods);
 
         public override ScoreProcessor CreateScoreProcessor() => new OsuScoreProcessor();
 
@@ -239,7 +237,7 @@ namespace osu.Game.Rulesets.Osu
 
         public override IConvertibleReplayFrame CreateConvertibleReplayFrame() => new OsuReplayFrame();
 
-        public override IRulesetConfigManager CreateConfig(SettingsStore settings) => new OsuRulesetConfigManager(settings, RulesetInfo);
+        public override IRulesetConfigManager CreateConfig(SettingsStore? settings) => new OsuRulesetConfigManager(settings, RulesetInfo);
 
         protected override IEnumerable<HitResult> GetValidHitResults()
         {
diff --git a/osu.Game.Rulesets.Taiko/TaikoRuleset.cs b/osu.Game.Rulesets.Taiko/TaikoRuleset.cs
index b145d8d74a..6beb83575d 100644
--- a/osu.Game.Rulesets.Taiko/TaikoRuleset.cs
+++ b/osu.Game.Rulesets.Taiko/TaikoRuleset.cs
@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using System.Linq;
@@ -37,7 +35,7 @@ namespace osu.Game.Rulesets.Taiko
 {
     public class TaikoRuleset : Ruleset, ILegacyRuleset
     {
-        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod> mods = null) => new DrawableTaikoRuleset(this, beatmap, mods);
+        public override DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod>? mods = null) => new DrawableTaikoRuleset(this, beatmap, mods);
 
         public override ScoreProcessor CreateScoreProcessor() => new TaikoScoreProcessor();
 
diff --git a/osu.Game.Tests/Visual/Editing/TimelineTestScene.cs b/osu.Game.Tests/Visual/Editing/TimelineTestScene.cs
index 19e297a08d..c2b1ba3aba 100644
--- a/osu.Game.Tests/Visual/Editing/TimelineTestScene.cs
+++ b/osu.Game.Tests/Visual/Editing/TimelineTestScene.cs
@@ -3,6 +3,7 @@
 
 #nullable disable
 
+using System.Diagnostics;
 using osu.Framework.Allocation;
 using osu.Framework.Audio;
 using osu.Framework.Bindables;
@@ -45,7 +46,10 @@ namespace osu.Game.Tests.Visual.Editing
             Dependencies.Cache(EditorBeatmap);
             Dependencies.CacheAs<IBeatSnapProvider>(EditorBeatmap);
 
-            Composer = playable.BeatmapInfo.Ruleset.CreateInstance().CreateHitObjectComposer().With(d => d.Alpha = 0);
+            Composer = playable.BeatmapInfo.Ruleset.CreateInstance().CreateHitObjectComposer();
+            Debug.Assert(Composer != null);
+
+            Composer.Alpha = 0;
 
             Add(new OsuContextMenuContainer
             {
diff --git a/osu.Game/Beatmaps/WorkingBeatmap.cs b/osu.Game/Beatmaps/WorkingBeatmap.cs
index eb914e61d4..0a51c843cd 100644
--- a/osu.Game/Beatmaps/WorkingBeatmap.cs
+++ b/osu.Game/Beatmaps/WorkingBeatmap.cs
@@ -280,12 +280,15 @@ namespace osu.Game.Beatmaps
                 }
             }
 
-            IBeatmapProcessor processor = rulesetInstance.CreateBeatmapProcessor(converted);
+            var processor = rulesetInstance.CreateBeatmapProcessor(converted);
 
-            foreach (var mod in mods.OfType<IApplicableToBeatmapProcessor>())
-                mod.ApplyToBeatmapProcessor(processor);
+            if (processor != null)
+            {
+                foreach (var mod in mods.OfType<IApplicableToBeatmapProcessor>())
+                    mod.ApplyToBeatmapProcessor(processor);
 
-            processor?.PreProcess();
+                processor.PreProcess();
+            }
 
             // Compute default values for hitobjects, including creating nested hitobjects in-case they're needed
             foreach (var obj in converted.HitObjects)
diff --git a/osu.Game/Online/API/APIMod.cs b/osu.Game/Online/API/APIMod.cs
index dcbaaea012..176f10975d 100644
--- a/osu.Game/Online/API/APIMod.cs
+++ b/osu.Game/Online/API/APIMod.cs
@@ -48,7 +48,7 @@ namespace osu.Game.Online.API
 
         public Mod ToMod(Ruleset ruleset)
         {
-            Mod resultMod = ruleset.CreateModFromAcronym(Acronym);
+            Mod? resultMod = ruleset.CreateModFromAcronym(Acronym);
 
             if (resultMod == null)
             {
diff --git a/osu.Game/Overlays/Changelog/ChangelogEntry.cs b/osu.Game/Overlays/Changelog/ChangelogEntry.cs
index fdeba3f304..4d034007b1 100644
--- a/osu.Game/Overlays/Changelog/ChangelogEntry.cs
+++ b/osu.Game/Overlays/Changelog/ChangelogEntry.cs
@@ -4,6 +4,7 @@
 #nullable disable
 
 using System;
+using System.Diagnostics;
 using System.Net;
 using System.Text.RegularExpressions;
 using osu.Framework.Allocation;
@@ -93,7 +94,7 @@ namespace osu.Game.Overlays.Changelog
                 t.Colour = entryColour;
             });
 
-            if (!string.IsNullOrEmpty(entry.Repository))
+            if (!string.IsNullOrEmpty(entry.Repository) && !string.IsNullOrEmpty(entry.GithubUrl))
                 addRepositoryReference(title, entryColour);
 
             if (entry.GithubUser != null)
@@ -104,17 +105,22 @@ namespace osu.Game.Overlays.Changelog
 
         private void addRepositoryReference(LinkFlowContainer title, Color4 entryColour)
         {
+            Debug.Assert(!string.IsNullOrEmpty(entry.Repository));
+            Debug.Assert(!string.IsNullOrEmpty(entry.GithubUrl));
+
             title.AddText(" (", t =>
             {
                 t.Font = fontLarge;
                 t.Colour = entryColour;
             });
+
             title.AddLink($"{entry.Repository.Replace("ppy/", "")}#{entry.GithubPullRequestId}", entry.GithubUrl,
                 t =>
                 {
                     t.Font = fontLarge;
                     t.Colour = entryColour;
                 });
+
             title.AddText(")", t =>
             {
                 t.Font = fontLarge;
diff --git a/osu.Game/Overlays/Settings/Sections/RulesetSection.cs b/osu.Game/Overlays/Settings/Sections/RulesetSection.cs
index a5f5810214..6f0b3c27a0 100644
--- a/osu.Game/Overlays/Settings/Sections/RulesetSection.cs
+++ b/osu.Game/Overlays/Settings/Sections/RulesetSection.cs
@@ -28,7 +28,7 @@ namespace osu.Game.Overlays.Settings.Sections
             {
                 try
                 {
-                    SettingsSubsection section = ruleset.CreateSettings();
+                    SettingsSubsection? section = ruleset.CreateSettings();
 
                     if (section != null)
                         Add(section);
diff --git a/osu.Game/Rulesets/ILegacyRuleset.cs b/osu.Game/Rulesets/ILegacyRuleset.cs
index a8cfed4866..f4b03baccd 100644
--- a/osu.Game/Rulesets/ILegacyRuleset.cs
+++ b/osu.Game/Rulesets/ILegacyRuleset.cs
@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 namespace osu.Game.Rulesets
 {
     public interface ILegacyRuleset
diff --git a/osu.Game/Rulesets/Ruleset.cs b/osu.Game/Rulesets/Ruleset.cs
index cb72a1f20f..4bc82a5f7e 100644
--- a/osu.Game/Rulesets/Ruleset.cs
+++ b/osu.Game/Rulesets/Ruleset.cs
@@ -1,13 +1,10 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Concurrent;
 using System.Collections.Generic;
 using System.Linq;
-using JetBrains.Annotations;
 using osu.Framework.Extensions;
 using osu.Framework.Extensions.EnumExtensions;
 using osu.Framework.Graphics;
@@ -100,7 +97,7 @@ namespace osu.Game.Rulesets
         /// Returns a fresh instance of the mod matching the specified acronym.
         /// </summary>
         /// <param name="acronym">The acronym to query for .</param>
-        public Mod CreateModFromAcronym(string acronym)
+        public Mod? CreateModFromAcronym(string acronym)
         {
             return AllMods.FirstOrDefault(m => m.Acronym == acronym)?.CreateInstance();
         }
@@ -108,7 +105,7 @@ namespace osu.Game.Rulesets
         /// <summary>
         /// Returns a fresh instance of the mod matching the specified type.
         /// </summary>
-        public T CreateMod<T>()
+        public T? CreateMod<T>()
             where T : Mod
         {
             return AllMods.FirstOrDefault(m => m is T)?.CreateInstance() as T;
@@ -122,7 +119,6 @@ namespace osu.Game.Rulesets
         /// then the proper behaviour is to return an empty enumerable.
         /// <see langword="null"/> mods should not be present in the returned enumerable.
         /// </remarks>
-        [ItemNotNull]
         public abstract IEnumerable<Mod> GetModsFor(ModType type);
 
         /// <summary>
@@ -202,10 +198,9 @@ namespace osu.Game.Rulesets
             return value;
         }
 
-        [CanBeNull]
-        public ModAutoplay GetAutoplayMod() => CreateMod<ModAutoplay>();
+        public ModAutoplay? GetAutoplayMod() => CreateMod<ModAutoplay>();
 
-        public virtual ISkin CreateLegacySkinProvider([NotNull] ISkin skin, IBeatmap beatmap) => null;
+        public virtual ISkin? CreateLegacySkinProvider(ISkin skin, IBeatmap beatmap) => null;
 
         protected Ruleset()
         {
@@ -225,7 +220,7 @@ namespace osu.Game.Rulesets
         /// <param name="beatmap">The beatmap to create the hit renderer for.</param>
         /// <param name="mods">The <see cref="Mod"/>s to apply.</param>
         /// <exception cref="BeatmapInvalidForRulesetException">Unable to successfully load the beatmap to be usable with this ruleset.</exception>
-        public abstract DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod> mods = null);
+        public abstract DrawableRuleset CreateDrawableRulesetWith(IBeatmap beatmap, IReadOnlyList<Mod>? mods = null);
 
         /// <summary>
         /// Creates a <see cref="ScoreProcessor"/> for this <see cref="Ruleset"/>.
@@ -251,7 +246,7 @@ namespace osu.Game.Rulesets
         /// </summary>
         /// <param name="beatmap">The <see cref="IBeatmap"/> to be processed.</param>
         /// <returns>The <see cref="IBeatmapProcessor"/>.</returns>
-        public virtual IBeatmapProcessor CreateBeatmapProcessor(IBeatmap beatmap) => null;
+        public virtual IBeatmapProcessor? CreateBeatmapProcessor(IBeatmap beatmap) => null;
 
         public abstract DifficultyCalculator CreateDifficultyCalculator(IWorkingBeatmap beatmap);
 
@@ -259,12 +254,11 @@ namespace osu.Game.Rulesets
         /// Optionally creates a <see cref="PerformanceCalculator"/> to generate performance data from the provided score.
         /// </summary>
         /// <returns>A performance calculator instance for the provided score.</returns>
-        [CanBeNull]
-        public virtual PerformanceCalculator CreatePerformanceCalculator() => null;
+        public virtual PerformanceCalculator? CreatePerformanceCalculator() => null;
 
-        public virtual HitObjectComposer CreateHitObjectComposer() => null;
+        public virtual HitObjectComposer? CreateHitObjectComposer() => null;
 
-        public virtual IBeatmapVerifier CreateBeatmapVerifier() => null;
+        public virtual IBeatmapVerifier? CreateBeatmapVerifier() => null;
 
         public virtual Drawable CreateIcon() => new SpriteIcon { Icon = FontAwesome.Solid.QuestionCircle };
 
@@ -272,13 +266,13 @@ namespace osu.Game.Rulesets
 
         public abstract string Description { get; }
 
-        public virtual RulesetSettingsSubsection CreateSettings() => null;
+        public virtual RulesetSettingsSubsection? CreateSettings() => null;
 
         /// <summary>
         /// Creates the <see cref="IRulesetConfigManager"/> for this <see cref="Ruleset"/>.
         /// </summary>
         /// <param name="settings">The <see cref="SettingsStore"/> to store the settings.</param>
-        public virtual IRulesetConfigManager CreateConfig(SettingsStore settings) => null;
+        public virtual IRulesetConfigManager? CreateConfig(SettingsStore? settings) => null;
 
         /// <summary>
         /// A unique short name to reference this ruleset in online requests.
@@ -314,7 +308,7 @@ namespace osu.Game.Rulesets
         /// for conversion use.
         /// </summary>
         /// <returns>An empty frame for the current ruleset, or null if unsupported.</returns>
-        public virtual IConvertibleReplayFrame CreateConvertibleReplayFrame() => null;
+        public virtual IConvertibleReplayFrame? CreateConvertibleReplayFrame() => null;
 
         /// <summary>
         /// Creates the statistics for a <see cref="ScoreInfo"/> to be displayed in the results screen.
@@ -322,7 +316,6 @@ namespace osu.Game.Rulesets
         /// <param name="score">The <see cref="ScoreInfo"/> to create the statistics for. The score is guaranteed to have <see cref="ScoreInfo.HitEvents"/> populated.</param>
         /// <param name="playableBeatmap">The <see cref="IBeatmap"/>, converted for this <see cref="Ruleset"/> with all relevant <see cref="Mod"/>s applied.</param>
         /// <returns>The <see cref="StatisticRow"/>s to display. Each <see cref="StatisticRow"/> may contain 0 or more <see cref="StatisticItem"/>.</returns>
-        [NotNull]
         public virtual StatisticRow[] CreateStatisticsForScore(ScoreInfo score, IBeatmap playableBeatmap) => Array.Empty<StatisticRow>();
 
         /// <summary>
@@ -375,13 +368,11 @@ namespace osu.Game.Rulesets
         /// <summary>
         /// Creates ruleset-specific beatmap filter criteria to be used on the song select screen.
         /// </summary>
-        [CanBeNull]
-        public virtual IRulesetFilterCriteria CreateRulesetFilterCriteria() => null;
+        public virtual IRulesetFilterCriteria? CreateRulesetFilterCriteria() => null;
 
         /// <summary>
         /// Can be overridden to add a ruleset-specific section to the editor beatmap setup screen.
         /// </summary>
-        [CanBeNull]
-        public virtual RulesetSetupSection CreateEditorSetupSection() => null;
+        public virtual RulesetSetupSection? CreateEditorSetupSection() => null;
     }
 }
diff --git a/osu.Game/Rulesets/RulesetConfigCache.cs b/osu.Game/Rulesets/RulesetConfigCache.cs
index 017214df61..ab44e86048 100644
--- a/osu.Game/Rulesets/RulesetConfigCache.cs
+++ b/osu.Game/Rulesets/RulesetConfigCache.cs
@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System;
 using System.Collections.Generic;
 using osu.Framework.Graphics;
@@ -18,7 +16,7 @@ namespace osu.Game.Rulesets
         private readonly RealmAccess realm;
         private readonly RulesetStore rulesets;
 
-        private readonly Dictionary<string, IRulesetConfigManager> configCache = new Dictionary<string, IRulesetConfigManager>();
+        private readonly Dictionary<string, IRulesetConfigManager?> configCache = new Dictionary<string, IRulesetConfigManager?>();
 
         public RulesetConfigCache(RealmAccess realm, RulesetStore rulesets)
         {
@@ -42,7 +40,7 @@ namespace osu.Game.Rulesets
             }
         }
 
-        public IRulesetConfigManager GetConfigFor(Ruleset ruleset)
+        public IRulesetConfigManager? GetConfigFor(Ruleset ruleset)
         {
             if (!IsLoaded)
                 throw new InvalidOperationException($@"Cannot retrieve {nameof(IRulesetConfigManager)} before {nameof(RulesetConfigCache)} has loaded");
diff --git a/osu.Game/Screens/Play/SpectatorPlayer.cs b/osu.Game/Screens/Play/SpectatorPlayer.cs
index 797787b194..68cc21fc1c 100644
--- a/osu.Game/Screens/Play/SpectatorPlayer.cs
+++ b/osu.Game/Screens/Play/SpectatorPlayer.cs
@@ -3,6 +3,7 @@
 
 #nullable disable
 
+using System.Diagnostics;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
 using osu.Framework.Screens;
@@ -84,6 +85,7 @@ namespace osu.Game.Screens.Play
             foreach (var frame in bundle.Frames)
             {
                 IConvertibleReplayFrame convertibleFrame = GameplayState.Ruleset.CreateConvertibleReplayFrame();
+                Debug.Assert(convertibleFrame != null);
                 convertibleFrame.FromLegacy(frame, GameplayState.Beatmap);
 
                 var convertedFrame = (ReplayFrame)convertibleFrame;
diff --git a/osu.Game/Tests/Rulesets/TestRulesetConfigCache.cs b/osu.Game/Tests/Rulesets/TestRulesetConfigCache.cs
index a80154c38e..cf637983d9 100644
--- a/osu.Game/Tests/Rulesets/TestRulesetConfigCache.cs
+++ b/osu.Game/Tests/Rulesets/TestRulesetConfigCache.cs
@@ -1,8 +1,6 @@
 // Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
 // See the LICENCE file in the repository root for full licence text.
 
-#nullable disable
-
 using System.Collections.Concurrent;
 using osu.Game.Rulesets;
 using osu.Game.Rulesets.Configuration;
@@ -14,8 +12,8 @@ namespace osu.Game.Tests.Rulesets
     /// </summary>
     public class TestRulesetConfigCache : IRulesetConfigCache
     {
-        private readonly ConcurrentDictionary<string, IRulesetConfigManager> configCache = new ConcurrentDictionary<string, IRulesetConfigManager>();
+        private readonly ConcurrentDictionary<string, IRulesetConfigManager?> configCache = new ConcurrentDictionary<string, IRulesetConfigManager?>();
 
-        public IRulesetConfigManager GetConfigFor(Ruleset ruleset) => configCache.GetOrAdd(ruleset.ShortName, _ => ruleset.CreateConfig(null));
+        public IRulesetConfigManager? GetConfigFor(Ruleset ruleset) => configCache.GetOrAdd(ruleset.ShortName, _ => ruleset.CreateConfig(null));
     }
 }