Mark the property as nullable and add some assert check.

osu.Game.Rulesets.Osu/Mods/OsuModAutopilot.cs

@@ -3,7 +3,9 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
+using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Graphics.Sprites;
 using osu.Framework.Input.StateChanges;
 using osu.Game.Graphics;
@@ -28,16 +30,20 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         public bool RestartOnFail => false;
 
-        private OsuInputManager inputManager;
+        private OsuInputManager? inputManager;
 
-        private IFrameStableClock gameplayClock;
+        private IFrameStableClock? gameplayClock;
 
-        private List<OsuReplayFrame> replayFrames;
+        private List<OsuReplayFrame>? replayFrames;
 
         private int currentFrame;
 
         public void Update(Playfield playfield)
         {
+            Debug.Assert(inputManager != null);
+            Debug.Assert(gameplayClock != null);
+            Debug.Assert(replayFrames != null);
+
             if (currentFrame == replayFrames.Count - 1) return;
 
             double time = gameplayClock.CurrentTime;

osu.Game.Rulesets.Osu/Mods/OsuModBlinds.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Diagnostics;
 using osu.Framework.Allocation;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Containers;
@@ -31,7 +32,7 @@ namespace osu.Game.Rulesets.Osu.Mods
         public override double ScoreMultiplier => UsesDefaultConfiguration ? 1.12 : 1;
         public override Type[] IncompatibleMods => new[] { typeof(OsuModFlashlight) };
 
-        private DrawableOsuBlinds blinds;
+        private DrawableOsuBlinds? blinds;
 
         public void ApplyToDrawableRuleset(DrawableRuleset<OsuHitObject> drawableRuleset)
         {
@@ -40,6 +41,8 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         public void ApplyToHealthProcessor(HealthProcessor healthProcessor)
         {
+            Debug.Assert(blinds != null);
+
             healthProcessor.Health.ValueChanged += health => { blinds.AnimateClosedness((float)health.NewValue); };
         }
 

osu.Game.Rulesets.Osu/Mods/OsuModFlashlight.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Linq;
 using osu.Framework.Bindables;
+using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Graphics;
 using osu.Framework.Input;
 using osu.Framework.Input.Events;
@@ -51,14 +52,14 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         public override float DefaultFlashlightSize => 180;
 
-        private OsuFlashlight flashlight;
+        private OsuFlashlight? flashlight;
 
         protected override Flashlight CreateFlashlight() => flashlight = new OsuFlashlight(this);
 
         public void ApplyToDrawableHitObject(DrawableHitObject drawable)
         {
             if (drawable is DrawableSlider s)
-                s.Tracking.ValueChanged += flashlight.OnSliderTrackingChange;
+                s.Tracking.ValueChanged += flashlight.AsNonNull().OnSliderTrackingChange;
         }
 
         private class OsuFlashlight : Flashlight, IRequireHighFrequencyMousePosition

osu.Game.Rulesets.Osu/Mods/OsuModMagnetised.cs

@@ -2,6 +2,7 @@
 // See the LICENCE file in the repository root for full licence text.
 
 using System;
+using System.Diagnostics;
 using osu.Framework.Bindables;
 using osu.Framework.Graphics.Sprites;
 using osu.Framework.Utils;
@@ -26,7 +27,7 @@ namespace osu.Game.Rulesets.Osu.Mods
         public override double ScoreMultiplier => 1;
         public override Type[] IncompatibleMods => new[] { typeof(OsuModAutopilot), typeof(OsuModWiggle), typeof(OsuModTransform), typeof(ModAutoplay), typeof(OsuModRelax), typeof(OsuModRepel) };
 
-        private IFrameStableClock gameplayClock;
+        private IFrameStableClock? gameplayClock;
 
         [SettingSource("Attraction strength", "How strong the pull is.", 0)]
         public BindableFloat AttractionStrength { get; } = new BindableFloat(0.5f)
@@ -74,6 +75,8 @@ namespace osu.Game.Rulesets.Osu.Mods
         {
             double dampLength = Interpolation.Lerp(3000, 40, AttractionStrength.Value);
 
+            Debug.Assert(gameplayClock != null);
+
             float x = (float)Interpolation.DampContinuously(hitObject.X, destination.X, dampLength, gameplayClock.ElapsedFrameTime);
             float y = (float)Interpolation.DampContinuously(hitObject.Y, destination.Y, dampLength, gameplayClock.ElapsedFrameTime);
 

osu.Game.Rulesets.Osu/Mods/OsuModNoScope.cs

@@ -4,6 +4,7 @@
 using System;
 using System.Linq;
 using osu.Framework.Bindables;
+using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Utils;
 using osu.Game.Beatmaps;
 using osu.Game.Configuration;
@@ -19,7 +20,7 @@ namespace osu.Game.Rulesets.Osu.Mods
     {
         public override string Description => "Where's the cursor?";
 
-        private PeriodTracker spinnerPeriods;
+        private PeriodTracker? spinnerPeriods;
 
         [SettingSource(
             "Hidden at combo",
@@ -41,7 +42,7 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         public void Update(Playfield playfield)
         {
-            bool shouldAlwaysShowCursor = IsBreakTime.Value || spinnerPeriods.IsInAny(playfield.Clock.CurrentTime);
+            bool shouldAlwaysShowCursor = IsBreakTime.Value || spinnerPeriods.AsNonNull().IsInAny(playfield.Clock.CurrentTime);
             float targetAlpha = shouldAlwaysShowCursor ? 1 : ComboBasedAlpha;
             playfield.Cursor.Alpha = (float)Interpolation.Lerp(playfield.Cursor.Alpha, targetAlpha, Math.Clamp(playfield.Time.Elapsed / TRANSITION_DURATION, 0, 1));
         }

osu.Game.Rulesets.Osu/Mods/OsuModRelax.cs

@@ -29,7 +29,7 @@ namespace osu.Game.Rulesets.Osu.Mods
         private bool isDownState;
         private bool wasLeft;
 
-        private OsuInputManager osuInputManager;
+        private OsuInputManager? osuInputManager;
 
         private ReplayState<OsuAction>? state;
         private double lastStateChangeTime;
@@ -44,6 +44,8 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         public void ApplyToPlayer(Player player)
         {
+            Debug.Assert(osuInputManager != null);
+
             if (osuInputManager.ReplayInputHandler != null)
             {
                 hasReplay = true;
@@ -132,6 +134,8 @@ namespace osu.Game.Rulesets.Osu.Mods
                     wasLeft = !wasLeft;
                 }
 
+                Debug.Assert(osuInputManager != null);
+
                 state?.Apply(osuInputManager.CurrentState, osuInputManager);
             }
         }

osu.Game.Rulesets.Osu/Mods/OsuModTarget.cs

@@ -3,8 +3,10 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using System.Linq;
 using osu.Framework.Bindables;
+using osu.Framework.Extensions.ObjectExtensions;
 using osu.Framework.Graphics;
 using osu.Framework.Graphics.Sprites;
 using osu.Framework.Utils;
@@ -94,11 +96,11 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         #region Private Fields
 
-        private ControlPointInfo controlPointInfo;
+        private ControlPointInfo? controlPointInfo;
 
-        private List<OsuHitObject> originalHitObjects;
+        private List<OsuHitObject>? originalHitObjects;
 
-        private Random rng;
+        private Random? rng;
 
         #endregion
 
@@ -158,7 +160,7 @@ namespace osu.Game.Rulesets.Osu.Mods
                 circle.ApproachCircle.Hide();
             }
 
-            using (circle.BeginAbsoluteSequence(startTime - controlPointInfo.TimingPointAt(startTime).BeatLength - undim_duration))
+            using (circle.BeginAbsoluteSequence(startTime - controlPointInfo.AsNonNull().TimingPointAt(startTime).BeatLength - undim_duration))
                 circle.FadeColour(Color4.White, undim_duration);
         }
 
@@ -200,6 +202,8 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         private IEnumerable<double> generateBeats(IBeatmap beatmap)
         {
+            Debug.Assert(originalHitObjects != null);
+
             double startTime = originalHitObjects.First().StartTime;
             double endTime = originalHitObjects.Last().GetEndTime();
 
@@ -228,6 +232,8 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         private void addHitSamples(IEnumerable<OsuHitObject> hitObjects)
         {
+            Debug.Assert(originalHitObjects != null);
+
             foreach (var obj in hitObjects)
             {
                 var samples = getSamplesAtTime(originalHitObjects, obj.StartTime);
@@ -240,6 +246,8 @@ namespace osu.Game.Rulesets.Osu.Mods
 
         private void fixComboInfo(List<OsuHitObject> hitObjects)
         {
+            Debug.Assert(originalHitObjects != null);
+
             // Copy combo indices from an original object at the same time or from the closest preceding object
             // (Objects lying between two combos are assumed to belong to the preceding combo)
             hitObjects.ForEach(newObj =>
@@ -276,7 +284,7 @@ namespace osu.Game.Rulesets.Osu.Mods
         {
             if (hitObjects.Count == 0) return;
 
-            float nextSingle(float max = 1f) => (float)(rng.NextDouble() * max);
+            float nextSingle(float max = 1f) => (float)(rng.AsNonNull().NextDouble() * max);
 
             const float two_pi = MathF.PI * 2;
 
@@ -357,6 +365,8 @@ namespace osu.Game.Rulesets.Osu.Mods
         /// <param name="time">The time to be checked.</param>=
         private bool isInsideBreakPeriod(IEnumerable<BreakPeriod> breaks, double time)
         {
+            Debug.Assert(originalHitObjects != null);
+
             return breaks.Any(breakPeriod =>
             {
                 var firstObjAfterBreak = originalHitObjects.First(obj => almostBigger(obj.StartTime, breakPeriod.EndTime));
@@ -372,6 +382,8 @@ namespace osu.Game.Rulesets.Osu.Mods
             int i = 0;
             double currentTime = timingPoint.Time;
 
+            Debug.Assert(controlPointInfo != null);
+
             while (!definitelyBigger(currentTime, mapEndTime) && ReferenceEquals(controlPointInfo.TimingPointAt(currentTime), timingPoint))
             {
                 beats.Add(Math.Floor(currentTime));