c# - 系统无效操作异常错误

Question

昨天我创建了两个表单，第一个表单监听客户端网络的请求，第二个表单是类似于MessageBox的表单警报（MessageBox自定义），但是当客户端向第一个表单发送连接请求时，表单第二个会显示，但点击任何按钮关闭表单后，我收到以下消息：

标题：System.InvalidOperationException 详细信息：撤消操作遇到的上下文与相应的 Set 操作中应用的上下文不同。可能的原因是在线程上设置了上下文并且未还原（撤消）。

第一个来自：

//Nhận các yêu cầu và kiểm tra và gửi phản hồi
    private void OnReceive(IAsyncResult ar)
    {
        try
        {
            EndPoint receivedFromEP = new IPEndPoint(IPAddress.Any, 0);

            //Get the IP from where we got a message.
            clientSocket.EndReceiveFrom(ar, ref receivedFromEP);

            //Convert the bytes received into an object of type Data.
            Data(byteData);

            //Act according to the received message.
            switch (cmdCommand)
            {
                //We have an incoming call.
                case Command.Invite:
                    {
                        if (bIsCallActive == false)
                        {
                            //We have no active call.                                
                            //Ask the user to accept the call or not.                                

                            DialogResult result = ThongBaoCoDienThoai.HienHopThoai(strName);                                

                            if (result == DialogResult.Yes)
                            {
                                //SendMessage(Command.OK, receivedFromEP);                                    
                                //Bắt đầu kết nôi
                                //**********************************GOI PHUONg THUC GUI NHAN DONG BO AM THANH****************                                    
                            }
                            else
                            {
                                //The call is declined. Send a busy response.
                               //SendMessage(Command.Busy, receivedFromEP);
                            }                        

                        }
                        else
                        {
                            //We already have an existing call. Send a busy response.
                            SendMessage(Command.Busy, receivedFromEP);
                        }
                        break;
                    }

                //OK is received in response to an Invite.
                case Command.OK:
                    {
                        //Start a call.                            
                        break;
                    }

                //Remote party is busy.
                case Command.Busy:
                    {
                        MessageBox.Show("User busy.", "VoiceChat", MessageBoxButtons.OK, MessageBoxIcon.Exclamation);
                        break;
                    }

                case Command.Bye:
                    {
                        //Check if the Bye command has indeed come from the user/IP with which we have
                        //a call established. This is used to prevent other users from sending a Bye, which
                        //would otherwise end the call.
                        if (receivedFromEP.Equals(otherPartyEP) == true)
                        {
                            //End the call.

                        }
                        break;
                    }
            }

            byteData = new byte[1024];
            //Get ready to receive more commands.
            clientSocket.BeginReceiveFrom(byteData, 0, byteData.Length, SocketFlags.None, ref receivedFromEP, new AsyncCallback(OnReceive), null);
        }
        catch (Exception ex)
        {
            MessageBox.Show("OnReceive: " + ex.Message, "Error");
        }
    }

第二种形式：

static ThongBaoCoDienThoai tbcdt;
    static DialogResult kq = DialogResult.No;
    private Timer thoiGian;         
    int henGioTat; //Hẹn thời gian tắt form
    SoundPlayer player; //Khởi tạo thiết bị phát (Loa)

    public ThongBaoCoDienThoai()
    {
        InitializeComponent();
        try
        {
            henGioTat = 30;
            thoiGian = new Timer();
            thoiGian.Interval = 1000;
            thoiGian.Enabled = true;
            thoiGian.Start();
            thoiGian.Tick += new EventHandler(this.thoiGian_Tick);

            //Hiển thị hộp thông báo ở góc màng hình dưới bên phải
            Rectangle r = Screen.PrimaryScreen.WorkingArea;
            this.StartPosition = FormStartPosition.Manual;
            this.Location = new Point(Screen.PrimaryScreen.WorkingArea.Width - this.Width, Screen.PrimaryScreen.WorkingArea.Height - this.Height);

            //Phát âm thành khi form được mở  
            player = new SoundPlayer(Properties.Resources.ringing);
            player.Load();
            player.PlayLooping();

        }
        catch (Exception ex)
        {
            MessageBox.Show("ThongBaoCoDienThoai_Load :" + ex.Message, "Error");
        }
    }

    public static DialogResult HienHopThoai(string tenNguoiGoi)
    {            
        try
        {
            tbcdt = new ThongBaoCoDienThoai();
            tbcdt.lbThongBao.Text = tenNguoiGoi + " đang gọi........";
            tbcdt.ShowDialog();                
        }
        catch (Exception ex)
        {
            MessageBox.Show("HienHopThoai: "+ex.Message,"Error");
        }
        return kq;
    }        

    private void btHuy_Click(object sender, EventArgs e)
    {
        tbcdt.thoiGian.Stop();
        tbcdt.thoiGian.Dispose();
        kq = DialogResult.No;
        tbcdt.Dispose();            
    }

    private void btNhan_Click(object sender, EventArgs e)
    {
        ThongBaoCoDienThoai tbcdt = new ThongBaoCoDienThoai();
        alertControl1.Show(tbcdt);
        tbcdt.thoiGian.Stop();
        tbcdt.thoiGian.Dispose();
        kq = DialogResult.Yes;
        tbcdt.Dispose();
    }        

    private void thoiGian_Tick(object sender, EventArgs e)
    {
        henGioTat--;

        if (henGioTat == 0)
        {
            tbcdt.thoiGian.Stop();
            tbcdt.thoiGian.Dispose();
            tbcdt.Dispose();
        }
    }

    private void ThongBaoCoDienThoai_FormClosing(object sender, FormClosingEventArgs e)
    {
        player.Stop();            
    }