Protect against potential nullref when exiting player (#7330)

Protect against potential nullref when exiting player

osu.Game/Screens/Play/Player.cs

@@ -562,7 +562,7 @@ namespace osu.Game.Screens.Play
 
             // GameplayClockContainer performs seeks / start / stop operations on the beatmap's track.
             // as we are no longer the current screen, we cannot guarantee the track is still usable.
-            GameplayClockContainer.StopUsingBeatmapClock();
+            GameplayClockContainer?.StopUsingBeatmapClock();
 
             fadeOut();
             return base.OnExiting(next);