From d9b5f235d88a28ba1577a94f479016b1ef81d397 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Tue, 10 Aug 2021 17:36:58 +0900
Subject: [PATCH 1/2] Add xmldoc explaining thread safety limitations of
 `IModelManager` "events"

---
 osu.Game/Database/IModelManager.cs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/osu.Game/Database/IModelManager.cs b/osu.Game/Database/IModelManager.cs
index 7f7e5565f1..8c314f1617 100644
--- a/osu.Game/Database/IModelManager.cs
+++ b/osu.Game/Database/IModelManager.cs
@@ -13,8 +13,16 @@ namespace osu.Game.Database
     public interface IModelManager<TModel>
         where TModel : class
     {
+        /// <summary>
+        /// A bindable which contains a weak reference to the last item that was updated.
+        /// This is not thread-safe and should be scheduled locally if consumed from a drawable component.
+        /// </summary>
         IBindable<WeakReference<TModel>> ItemUpdated { get; }
 
+        /// <summary>
+        /// A bindable which contains a weak reference to the last item that was removed.
+        /// This is not thread-safe and should be scheduled locally if consumed from a drawable component.
+        /// </summary>
         IBindable<WeakReference<TModel>> ItemRemoved { get; }
     }
 }

From b121d95400727bedb1c6d929d8ed5a741e904399 Mon Sep 17 00:00:00 2001
From: Dean Herbert <pe@ppy.sh>
Date: Tue, 10 Aug 2021 17:37:24 +0900
Subject: [PATCH 2/2] Avoid potential null reference exception in
 `OnlinePlayBeatmapAvailabilityTracker`

---
 osu.Game/Online/Rooms/OnlinePlayBeatmapAvailabilityTracker.cs | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/osu.Game/Online/Rooms/OnlinePlayBeatmapAvailabilityTracker.cs b/osu.Game/Online/Rooms/OnlinePlayBeatmapAvailabilityTracker.cs
index 72ea84d4a8..86879ba245 100644
--- a/osu.Game/Online/Rooms/OnlinePlayBeatmapAvailabilityTracker.cs
+++ b/osu.Game/Online/Rooms/OnlinePlayBeatmapAvailabilityTracker.cs
@@ -59,8 +59,8 @@ namespace osu.Game.Online.Rooms
 
         protected override bool VerifyDatabasedModel(BeatmapSetInfo databasedSet)
         {
-            int? beatmapId = SelectedItem.Value.Beatmap.Value.OnlineBeatmapID;
-            string checksum = SelectedItem.Value.Beatmap.Value.MD5Hash;
+            int? beatmapId = SelectedItem.Value?.Beatmap.Value.OnlineBeatmapID;
+            string checksum = SelectedItem.Value?.Beatmap.Value.MD5Hash;
 
             var matchingBeatmap = databasedSet.Beatmaps.FirstOrDefault(b => b.OnlineBeatmapID == beatmapId && b.MD5Hash == checksum);