Quick fixes.

- Fixed a crash when closing the application in the ranking screen when viewing a score.
- Fixed a minor bug where OsuGroupList fields were not being erased upon restart.
- Dim the track volume by 50% when viewing a score.

Signed-off-by: Jeffrey Han <itdelatrisu@gmail.com>

src/itdelatrisu/opsu/states/MainMenu.java

@@ -249,7 +249,7 @@ public class MainMenu extends BasicGameState {
 		// window focus change: increase/decrease theme song volume
 		if (MusicController.isThemePlaying() &&
 		    MusicController.isTrackDimmed() == container.hasFocus())
-				MusicController.toggleTrackDimmed();
+				MusicController.toggleTrackDimmed(0.33f);
 
 		// fade in background
 		if (bgAlpha < 0.9f) {