Merge pull request #9943 from peppy/fix-double-storyboard-sample-dispose-crash

Fix potential crash if disposing a DrawableStoryboardSample twice

osu.Game/Storyboards/Drawables/DrawableStoryboardSample.cs

@@ -76,6 +76,8 @@ namespace osu.Game.Storyboards.Drawables
         protected override void Dispose(bool isDisposing)
         {
             Channel?.Stop();
+            Channel = null;
+
             base.Dispose(isDisposing);
         }
     }