Fix a possible crash in EventBus

src/main/java/net/minecraftforge/fml/common/eventhandler/EventBus.java

@@ -121,6 +121,8 @@ public class EventBus implements IEventExceptionHandler
     public void unregister(Object object)
     {
         ArrayList<IEventListener> list = listeners.remove(object);
+        if(list == null)
+            return;
         for (IEventListener listener : list)
         {
             ListenerList.unregisterAll(busID, listener);