Fix SimulatorWatcher for non watched Simulator

BauSystem_Main/src/de/steamwar/bausystem/features/simulator2/SimulatorWatcher.java

@@ -58,8 +58,10 @@ public class SimulatorWatcher {
 
     public void update(Simulator simulator) {
         REntityServer rEntityServer = entityServers.get(simulator);
+        if (rEntityServer != null) {
             rEntityServer.getEntities().forEach(REntity::die);
             createSim(rEntityServer, simulator);
+        }
 
         new ArrayList<>(watchers.values()).forEach(simulatorRunnablePair -> {
             if (simulatorRunnablePair.getKey() == simulator) {