docs: @Override
      public void unregisterBroadcastListener(String broadcastName, BroadcastListener broadcastListener) {
          List<BroadcastListener> listeners = broadcastListeners.get(broadcastName);
          if (listeners == null) {
              LOG.error("trying to unregister a listener for broadcast \"" + broadcastName
                      + "\" that was never registered");
              return;
          }
          synchronized (listeners) {
              boolean success = listeners.remove(broadcastListener);
              if (!success) {
                  LOG.error("trying to unregister a listener for broadcast \"" + broadcastName
                          + "\" that was never registered");
                  return;
              }
          }
      }