System Invalid Operation Exception error

//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.
                            //**********************************GOI PHUON THUC GUI NHAN DONG BO AM THANH****************
                            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");
            }
        }

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();            
        }