Fix mode null

Signed-off-by: Lixfel <agga-games@gmx.de>

FightSystem_Core/src/de/steamwar/fightsystem/listener/LeaveableArena.java

@@ -76,7 +76,9 @@ public class LeaveableArena implements Listener {
         if(inArena && !spectator) {
             markInArena(player);
         } else if(!inArena && spectator) {
-            player.setGameMode(spectatorsInArena.remove(player));
+            GameMode mode = spectatorsInArena.remove(player);
+            if(mode != null)
+                player.setGameMode(mode);
         }
     }