How to close this service? The close method has no effect

[lalLAL95828]

  @override
  void initState() {
    super.initState();
    shelf_io.serve(handler, '10.0.2.15',3534).then((server) {
        _httpServer = server;
        setState(() {});
      }).catchError((err){
        print(err);
        setState(() {});
      });
  }

When I create a connection in initstate and then execute the close method in dispose, the service that the client has connected will not be disconnected

@override
  void dispose() {
    _httpServer?.close(force:true);
    super.dispose();
  }