code: private List<ResourceBuilderModel> getRegisteredListForEvent(String event) {
          if(eventSubscribers.containsKey(event)) {
              return eventSubscribers.get(event);
          } else {
              LinkedList<ResourceBuilderModel> tempList = new LinkedList<>();
              eventSubscribers.put(event, tempList);
              return tempList;
          }
      }
docs: returns a list of all the ResourceBuilders listening to the Event-ID
  @param event the eventID
  @return a List of ResourceBuilders