C#

System.InvalidOperationException: Nullable object must have a value.

Hi Guys,
I am not expert and will be glad to get some help. I wrote software, where I am using Barcode reader, com port, in number of Windows forms. I am switching receive data on Form Activation and disabling it on Form deactivated. it has worked well for number of months, but now I am occasionally get error, Com port is not accessible. Received I use Invoke.
<pre>        private void CheckForUsbDevice()
        {

            string[] ports = SerialPort.GetPortNames();

            // Display each port name to the console.
            foreach (string port in ports)
            {
                if ((port == "COM3") || (port == "COM4"))
                {
                    if (AcdStart.mySerialPort == null)
                    {
                        AcdStart.mySerialPort = new SerialPort(port)
                        {
                            BaudRate = 9600,
                            Parity = Parity.None,
                            StopBits = StopBits.One,
                            DataBits = 8,
                            Handshake = Handshake.None,
                            RtsEnable = true
                        };
                    }
                         AcdStart.mySerialPort.DataReceived += new SerialDataReceivedEventHandler(DataReceivedHandler);
                    if (!AcdStart.mySerialPort.IsOpen)
                    {
                        AcdStart.mySerialPort.Close();
                        AcdStart.mySerialPort.Open();
                    }
                        PortTextBox.Text = (string.Format("Port {0}", port));
                    break;
                }
            }
        }

private void RxPurchaseOrder_Deactivate(object sender, System.EventArgs e)
{
    if ((AcdStart.mySerialPort != null) && (AcdStart.mySerialPort.IsOpen))
        AcdStart.mySerialPort.DiscardInBuffer();

    if (AcdStart.mySerialPort != null)
    {
        AcdStart.mySerialPort.DataReceived -= new SerialDataReceivedEventHandler(DataReceivedHandler);
        AcdStart.mySerialPort.Close();
    }
    AcdStart.mySerialPort = null;
    PauseNow = true;
    this.BackColor = Color.LightSalmon;
}

private void RxPurchaseOrder_Activated(object sender, EventArgs e)
{
    if ((AcdStart.mySerialPort != null) && (AcdStart.mySerialPort.IsOpen))
    {
        AcdStart.mySerialPort.DiscardInBuffer();
        if (AcdStart.mySerialPort.IsOpen)
            AcdStart.mySerialPort.Close();


        AcdStart.mySerialPort = null;
    }
    PortTextBox.Text = "";
    CheckForUsbDevice();
    PauseNow = false;
    this.BackColor = Color.LightSteelBlue;
}