how to send grid row data after the multiple selection

Question

I want to send a single mail  after of multiple selection of check boxes from grid view data row.
I putted the following code :
protected void Button1_Click(object sender, EventArgs e)
{


// StringBuilder object
StringBuilder str = new StringBuilder();


// Select the checkboxes from the GridView control
for (int i = 0; i < GridView1.Rows.Count; i++)
{
GridViewRow row = GridView1.Rows[i];
bool isChecked = ((CheckBox)row.FindControl("chkSelect")).Checked;
//chkSelect.Checked = Convert.ToBoolean(Session["a"]);


if (isChecked)
{
// Column 2 is the Hoarding_ID column
str.Append(GridView1.Rows[i].Cells[0].Text);
}
}
// prints out the result
Response.Write(str.ToString());












string a = GridView1.SelectedRow.Cells[2].Text;
Session["a"] = a.ToString();


con.Open();
SqlDataAdapter da = new SqlDataAdapter("select email_id from Hoarding_buyers where username = '" + Session["b"].ToString() + "'", con);
DataSet ds = new DataSet();
da.Fill(ds);
DataTable dt = ds.Tables[0];
con.Close();


if (Session["b"] != null)
{


System.Web.Mail.MailMessage objMM = new System.Web.Mail.MailMessage();
objMM.To = dt.Rows[0][0].ToString();
objMM.From = "[email protected]";


objMM.BodyFormat = System.Web.Mail.MailFormat.Text;
objMM.Priority = System.Web.Mail.MailPriority.High;
objMM.Subject = "HoardingIndia";
objMM.Body = "Dear Sir/Madam,\r\n\r\n" +
"Thank You for Contacting Us. \r\n" +
"Our Marketing Team will contact you soon.\r\n\r\n" +
"Thanks & regards \r\n" +
"Hoardingindia.co.in \r\n" +
"011-22239165, [phone] \r\n\r\n";


objMM.Fields["http://schemas.microsoft.com/cdo/configuration/smtpserver"] = "smtp.gmail.com";
objMM.Fields["http://schemas.microsoft.com/cdo/configuration/smtpserverport"] = 465;
objMM.Fields["http://schemas.microsoft.com/cdo/configuration/sendusing"] = 2;
objMM.Fields["http://schemas.microsoft.com/cdo/configuration/smtpauthenticate"] = 1;
objMM.Fields["http://schemas.microsoft.com/cdo/configuration/sendusername"] = "hoardingindia2012";
objMM.Fields["http://schemas.microsoft.com/cdo/configuration/sendpassword"] = "hoardingindia2012";
objMM.Fields["http://schemas.microsoft.com/cdo/configuration/smtpusessl"] = true;
System.Web.Mail.SmtpMail.SmtpServer = "smtp.gmail.com";
System.Web.Mail.SmtpMail.Send(objMM);


con.Open();
SqlDataAdapter da1 = new SqlDataAdapter("select * from Hoarding_buyers where username = '" + Session["b"].ToString() + "'", con);
DataSet ds1 = new DataSet();
da1.Fill(ds1);
DataTable dt1 = ds1.Tables[0];
con.Close();


con.Open();
SqlDataAdapter da2 = new SqlDataAdapter("select * from providers_post where h_id = '" + Session["a"].ToString() + "'", con);
DataSet ds2 = new DataSet();
da2.Fill(ds2);
DataTable dt2 = ds2.Tables[0];
con.Close();


con.Open();
SqlDataAdapter da3 = new SqlDataAdapter("select * from providers where provider_user_name in (select Provider_user_name from providers_post where h_id = '" + Session["a"].ToString() + "')", con);
DataSet ds3 = new DataSet();
da3.Fill(ds3);
DataTable dt3 = ds3.Tables[0];
con.Close();




System.Web.Mail.MailMessage objMM1 = new System.Web.Mail.MailMessage();
objMM1.To = "[email protected]";
objMM1.From = "[email protected]";


objMM1.BodyFormat = System.Web.Mail.MailFormat.Text;
objMM1.Priority = System.Web.Mail.MailPriority.High;
objMM1.Subject = "HoardingIndia";
objMM1.Body = "Dear Sir/Madam,\r\n\r\n" +


"We have received the details from hoarding buyer. \r\n\r\n" +


"Name :" + dt1.Rows[0][1].ToString() + "\r\n" +
"Email :" + dt1.Rows[0][10].ToString() + "\r\n" +
"Contact No.:" + dt1.Rows[0][3].ToString() + "\r\n\r\n" +


"-----------------------------------------------------------------------" + "\r\n\r\n" +


"Following are the hoarding details. \r\n\r\n" +


"Hoarding ID:" + Session["a"].ToString() + "\r\n\r\n" +
"Address :" + dt2.Rows[0][1].ToString() + "\r\n" +
"LandMark :" + dt2.Rows[0][6].ToString() + "\r\n" +
"State :" + dt2.Rows[0][4].ToString() + "\r\n" +
"District :" + dt2.Rows[0][5].ToString() + "\r\n" +
"Pin Code :" + dt2.Rows[0][8].ToString() + "\r\n" +
"Size :" + dt2.Rows[0][3].ToString() + "\r\n" +
"From Date:" + dt2.Rows[0][9].ToString() + "\r\n" +
"To Date :" + dt2.Rows[0][14].ToString() + "\r\n\r\n" +
"Hoarding Type:" + dt2.Rows[0][11].ToString() + "\r\n" +
"Lighting Type :" + dt2.Rows[0][15].ToString() + "\r\n" +
"Hoarding Cost:" + dt2.Rows[0][7].ToString() + "\r\n\r\n" +






"-----------------------------------------------------------------------" + "\r\n\r\n" +


"Following are the Hoarding owner's details in which buyer is intereated: \r\n\r\n" +


"Name :" + dt3.Rows[0][1].ToString() + "\r\n" +
"Email Id :" + dt3.Rows[0][6].ToString() + "\r\n" +
"Contact No.: " + dt3.Rows[0][2].ToString() + "\r\n\r\n\r\n";


objMM1.Fields["http://schemas.microsoft.com/cdo/configuration/smtpserver"] = "smtp.gmail.com";
objMM1.Fields["http://schemas.microsoft.com/cdo/configuration/smtpserverport"] = 465;
objMM1.Fields["http://schemas.microsoft.com/cdo/configuration/sendusing"] = 2;
objMM1.Fields["http://schemas.microsoft.com/cdo/configuration/smtpauthenticate"] = 1;
objMM1.Fields["http://schemas.microsoft.com/cdo/configuration/sendusername"] = "hoardingindia2012";
objMM1.Fields["http://schemas.microsoft.com/cdo/configuration/sendpassword"] = "hoardingindia2012";
objMM1.Fields["http://schemas.microsoft.com/cdo/configuration/smtpusessl"] = true;
System.Web.Mail.SmtpMail.SmtpServer = "smtp.gmail.com";
System.Web.Mail.SmtpMail.Send(objMM1);


//Response.Write("<script>alert('Thank you,we will contact you soon.')</script>");
Response.Redirect("search.aspx");
}


} 

but the above code is throwing following error please help me:

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error: 

Line 437: { Line 438: Line 439: string a = GridView1.SelectedRow.Cells[2].ToString(); Line 440: Session["a"] = a.ToString(); Line 441:

 earlier it was sending a mail by selecting single row data but now i putted the check boxes after this its not working