Merge pull request #838 from MrTheMake/playsongselect-exit-fix

Unregister OnBeatmapChanged() when exiting song selection

osu.Game/Screens/OsuScreen.cs

@@ -131,7 +131,15 @@ namespace osu.Game.Screens
                     Background.Exit();
             }
 
-            return base.OnExiting(next);
+            if (base.OnExiting(next))
+                return true;
+
+            // while this is not necessary as we are constructing our own bindable, there are cases where
+            // the GC doesn't run as fast as expected and this is triggered post-exit.
+            // added to resolve https://github.com/ppy/osu/issues/829
+            beatmap.ValueChanged -= OnBeatmapChanged;
+
+            return false;
         }
     }
 }