have problem with system.NullReferenceException error ...please help

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;

namespace student
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
        }

        protected void clicked(object sender, EventArgs e)
        {
            try
            {
                SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["stdConnectionStrings"].ConnectionString);
                con.Open();
                string user = "select count(*)from the stdtable where std_name='" + TextBox1.Text + "'";
                SqlCommand com = new SqlCommand(user, con);
                int a = Convert.ToInt32(com.ExecuteScalar());
                if (a == 1)
                {
                    Response.Write("user already exist");
                }
                string insertQuery = "insert into stdtable (std_name,roll,std_address) values (@std_name,@roll,@std_address)";
                com.Parameters.AddWithValue("@std_name", TextBox1.Text);
                com.Parameters.AddWithValue("roll", TextBox2.Text);
                com.Parameters.AddWithValue("@std_address", TextBox3.Text);
                com.ExecuteNonQuery();
                Response.Redirect("stdadmin.aspx");
                Response.Write("registration succescesfull...");
                con.Close();
            }
            catch (Exception ex)
            {
                Response.Write("error: " + ex.ToString());
            }
        }
    }
}