From 6be41e497a2a9556ac4fc8334e9a5bc90fe4193a Mon Sep 17 00:00:00 2001
From: smoogipoo <smoogipoo@smgi.me>
Date: Wed, 16 Jun 2021 11:25:00 +0900
Subject: [PATCH] Fix possible nullref in difficulty recommender

---
 osu.Game/Beatmaps/DifficultyRecommender.cs | 18 ++++++++++++++----
 1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/osu.Game/Beatmaps/DifficultyRecommender.cs b/osu.Game/Beatmaps/DifficultyRecommender.cs
index 340c47d89b..ca910e70b8 100644
--- a/osu.Game/Beatmaps/DifficultyRecommender.cs
+++ b/osu.Game/Beatmaps/DifficultyRecommender.cs
@@ -101,10 +101,20 @@ namespace osu.Game.Beatmaps
         /// Rulesets ordered descending by their respective recommended difficulties.
         /// The currently selected ruleset will always be first.
         /// </returns>
-        private IEnumerable<RulesetInfo> orderedRulesets =>
-            recommendedDifficultyMapping
-                .OrderByDescending(pair => pair.Value).Select(pair => pair.Key).Where(r => !r.Equals(ruleset.Value))
-                .Prepend(ruleset.Value);
+        private IEnumerable<RulesetInfo> orderedRulesets
+        {
+            get
+            {
+                if (LoadState < LoadState.Ready || ruleset.Value == null)
+                    return Enumerable.Empty<RulesetInfo>();
+
+                return recommendedDifficultyMapping
+                       .OrderByDescending(pair => pair.Value)
+                       .Select(pair => pair.Key)
+                       .Where(r => !r.Equals(ruleset.Value))
+                       .Prepend(ruleset.Value);
+            }
+        }
 
         private void onlineStateChanged(ValueChangedEvent<APIState> state) => Schedule(() =>
         {