Fix TypeMappers.arenaMapTypeMapper

VelocityCore/src/de/steamwar/velocitycore/commands/TypeMappers.java

@@ -50,6 +50,7 @@ public class TypeMappers {
             @Override
             public ArenaMode map(Chatter sender, PreviousArguments previousArguments, String s) {
                 ArenaMode arenaMode = ArenaMode.getByChat(s);
+                if (arenaMode == null) return null;
                 if (arenaMode.isHistoric() != historic) return null;
                 if (!arenaMode.isActive()) return null;
                 return arenaMode;