docs: synchronized void deleteListener(final AbstractEventListener<?> listener) {
          final String eventType = listener.getEventType();
          if (null == eventType) {
              throw new NullPointerException();
          }
  
          final List<AbstractEventListener<?>> listenerList = listeners.get(eventType);
          if (null != listenerList) {
              listenerList.remove(listener);
          }
      }