code: @Override
      public void close(FutureCallback<Void> callback) throws IllegalStateException {
          assertOpen();
  
          Future<?> future = this.eventGroup.shutdownGracefully(0L, 5L, TimeUnit.SECONDS);
          future.addListener(new NetworkManagerVoidFutureCallback(callback));
      }
docs: {@inheritDoc}
  
  @throws IllegalStateException If manager is already closed.