Merge pull request #18586 from peppy/skin-editor-null-ref-on-game-exit

Fix potential null reference in skin editor if target screen is null (during exit)

osu.Game/Skinning/Editor/SkinEditorOverlay.cs

@@ -127,6 +127,9 @@ namespace osu.Game.Skinning.Editor
 
         private void setTarget(OsuScreen target)
         {
+            if (target == null)
+                return;
+
             Debug.Assert(skinEditor != null);
 
             if (!target.IsLoaded)