Why UDPclient isn't receiving nomore after passing it in a new form?

From an higher level, the architecture of my application is this:

public class NetworkService
{
        // Other things

        private UdpClient receiving_client_;
        private UdpClient sending_client_;

        private Thread login_thread_;
        private Thread receiving_thread_;

    public NetworkService(
        int Port,
        string MyNickname)
    {
        this.port_ = Port;
        this.my_nickname_ = MyNickname;

        receiving_client_ = new UdpClient(port_);

        receiving_thread_ = new Thread(Receiver);
        receiving_thread_.IsBackground = true;
        receiving_thread_.Start();
    }

    // Other things

    private void Receiver()
    {
        IPEndPoint endPoint = new IPEndPoint(IPAddress.Any, port_);

        while (true)
        {
            byte[] byte_data = receiving_client_.Receive(ref endPoint);
            HandlePacket(data);
        }
    }
}

From the other side, I have 2 forms: Launcher and Lobby.

Inside Launcher I initialize a NetworkService and I use to contact another IP, when it answers an event is raised and Lobby form is created. Until here everything works fine: I receive and send all the packets correctly.

How I create the other form:

  Lobby lobby = new Lobby(ref network_service_, other_peer_ip_);
  lobby.ShowDialog();

public Lobby(ref NetworkService NetworkService, string OtherPeerIp)
    {
        Application.EnableVisualStyles();
        InitializeComponent();

        network_service_ = NetworkService;
        this.my_nickname_ = network_service_.my_nickname_;

        network_service_.OnChatMessageReceived += NewChatMessageReceived;
        network_service_.OnConnectionEstabilished += ConnectionEstabilished;
   }

The problem is that once the new form is loaded, i don't receive packets anymore! I send them, I checked with Wireshark. It seems that the receiving thread don't do anything, but I also checked that it was alive and it is.

I wonder if the problem is caused by the creation of the new form and the passing of NetworkService to it.