Prevent ConcurrentModificatonException

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

FightSystem_Core/src/de/steamwar/fightsystem/countdown/Countdown.java

@@ -82,7 +82,7 @@ public abstract class Countdown {
                 smallestTime = countdown.time;
         }
 
-        for(Countdown countdown : currentCountdowns){
+        for(Countdown countdown : new ArrayList<>(currentCountdowns)) {
             if(countdown.time - smallestTime <= 1)
                 countdown.prepareFinish();
             countdown.time -= smallestTime;