Remove engine references to MAINMENU_BG. Don't push the menu background onto the window list. Crashes whenever a delegate tries to open MAINMENU_BG.

OpenRA.Mods.Cnc/Missions/Gdi01Script.cs

@@ -59,6 +59,8 @@ namespace OpenRA.Mods.Cnc
 			{
 				Sound.StopMusic();
 				Game.Disconnect();
+				Widget.CloseWindow();
+				Widget.LoadWidget("MENU_BACKGROUND");
 			})));
 		}
 		
@@ -75,6 +77,8 @@ namespace OpenRA.Mods.Cnc
 			{
 				Sound.StopMusic();
 				Game.Disconnect();
+				Widget.CloseWindow();
+				Widget.LoadWidget("MENU_BACKGROUND");
 			})));
 		}