Sending SMS in ASP.NET gets this System.NullReferenceException

Question

I am trying to send SMS message using a GSM modem in ASP.NET. I tried using this code in my .NET application it works. But when I ran it in a Web app. Doesnt work.

I am using GSMComm library.

Whenever Im tryin to send a message I get this error:

Exception thrown:

'System.NullReferenceException' in App_Web_pqr1nllj.dll

Here is my code:

GsmCommMain comm;

protected void Connect_Click(object sender, EventArgs e)
{
    if(ComPort.Text == "")
    {
        return;
    }
    comm = new GsmCommMain(ComPort.Text, 9600, 150);

    bool retry;
    do
    {
        retry = false;
        try
        {
            comm.Open();
            string message = "Success!";
            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            sb.Append("<script type = 'text/javascript'>");
            sb.Append("window.onload = function(){");
            sb.Append("alert('");
            sb.Append(message);
            sb.Append("')};");
            sb.Append("</script>");

            ClientScript.RegisterStartupScript(this.GetType(), "alert", sb.ToString());
            retry = true;
        }
        catch(GsmComm.GsmCommunication.CommException)
        {
            string message = "Gsm not available";
            System.Text.StringBuilder sb = new System.Text.StringBuilder();
            sb.Append("<script type = 'text/javascript'>");
            sb.Append("window.onload = function(){");
            sb.Append("alert('");
            sb.Append(message);
            sb.Append("')};");
            sb.Append("</script>");

            ClientScript.RegisterStartupScript(this.GetType(), "alert", sb.ToString());
            return;
        }
    } while(retry);
}

protected void Send_Click(object sender, EventArgs e)
{
    try
    {
        SmsSubmitPdu pdu;
        //byte dcs = (byte)DataCodingScheme.GeneralCoding.Alpha7BitDefault;
        pdu = new SmsSubmitPdu(Message.Text, Number.Text);

        comm.SendMessage(pdu);
    }
    catch(Exception)
    {
        string message = "Sending failed!";
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.Append("<script type = 'text/javascript'>");
        sb.Append("window.onload = function(){");
        sb.Append("alert('");
        sb.Append(message);
        sb.Append("')};");
        sb.Append("</script>");

        ClientScript.RegisterStartupScript(this.GetType(), "alert", sb.ToString()); 
    } 
}

When I click the "send button", it gives an error in the debug panel in my visual studio.

How can I fix this error?