Object reference not set to an instance of an object.

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.Data.SqlClient;





//public class fields
//{
// public string _StudentName { get; set; }
// public int _RollNo { get; set; }
// public int _C { get; set; }
// public int _CPlusplus { get; set; }
// public int _Csharp { get; set; }
// public int _ASP { get; set; }
// int _TotalMarks { get; set; }
// double _Percentage { get; set; }
// float num3 { get; set; }
// float num8 { get; set; }

//}

public partial class _Default : System.Web.UI.Page
{


protected void Page_Load(object sender, EventArgs e)
{

SqlConnection con = new SqlConnection(@"Data Source=RMM-PC-343\SQLEXPRESS1;Initial Catalog=Student_Result;Integrated Security=True");
SqlCommand cmd = new SqlCommand("Select * from StudentResult", con);

cmd.CommandType = CommandType.Text;
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
GridView1.DataSource = ds.Tables[0];
GridView1.DataBind();

if (!IsPostBack)
{
BindStudentResult();
}
}

private void BindStudentResult()
{

SqlConnection con = new SqlConnection(@"Data Source=RMM-PC-343\SQLEXPRESS1;Initial Catalog=Student_Result;Integrated Security=True");
con.Open();
SqlCommand cmd = new SqlCommand("Select * from StudentResult", con);

SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
con.Close();


}






//private void bindgrid()
//{
// SqlConnection con = new SqlConnection(@"Data Source=RMM-PC-343\SQLEXPRESS1;Initial Catalog=Student_Result;Integrated Security=True");


// ds.Clear();
// con.ConnectionString//("data source=.\SQLEXPRESS;Integrated Security=SSPI");
// cmd.Connection=con;
// con.Open();

// string insert="insert into StudentResult(Studentname,Rollno,Cmarks,C++marks,Asp.netmarks,Marksobtained,totalmarks,Percentage)values("'@Studentname'",@Rollno,@Cmarks,@C++marks,@Asp.netmarks,@marksobtained,@totalmarks,@Percentage)";
// cmd=new SqlCommand(insert,con);
// cmd.ExecuteNonQuery();
// con.Close();

//}
DataTable dt;



public void BtnSubmit_Click(object sender, EventArgs e)


{

SqlConnection con = new SqlConnection(@"Data Source=RMM-PC-343\SQLEXPRESS1;Initial Catalog=Student_Result;Integrated Security=True");

con.Open();
calculatetotalmarks();
calculatepercentage();
string insrt = "insert into StudentResult values('" + TxtStudentname.Text + "','" + TxtRollno.Text + "','" + TxtC.Text + "','" + TxtCplusplus.Text + "','" + TxtCsharp.Text + "','" + TxtAspdotnet.Text + "','" + Txtmarksobtained.Text + "','" + Txttotalmarks.Text + "','" + TxtPercentage.Text + "')";
SqlCommand cmd = new SqlCommand(insrt,con);
cmd.CommandType = CommandType.Text;
// //SqlDataAdapter da = new SqlDataAdapter();
// DataSet ds = new DataSet();
//// da.Fill(ds);
// GridView1.DataSource = dt;
// GridView1.DataBind();

//SqlDataAdapter da = new SqlDataAdapter();
// DataSet ds = new DataSet();
// //da.Fill(ds);
//GridView1.DataSource = dt;
//GridView1.DataBind();
cmd.ExecuteNonQuery();
con.Close();

//DataSet ds= new DataSet();
//SqlDataAdapter da= new SqlDataAdapter ();
// da.Fill(ds);
//con.Close();


//con.open();
//cmd.parameters.AddWithValue("StudentName",TxtStudentname.Text);
//cmd.parameters.AddWithValue("RollNo",TxtRollno.Text);
//cmd.parameters.AddWithValue("Cmarks