"Object reference not set to an instance of an object."

The Following is the Code...



using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Drawing;
using System.Data;


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

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

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
BindEmployeeDetails();
}
}

private void BindEmployeeDetails()
{
con.Open();
SqlCommand cmd = new SqlCommand("Select * from Employees_Details", con);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
con.Close();
if (ds.Tables[0].Rows.Count > 0)
{
gvDetails.DataSource = ds;
gvDetails.DataBind();
}
else
{
ds.Tables[0].Rows.Add(ds.Tables[0].NewRow());
gvDetails.DataSource = ds;
gvDetails.DataBind();
int coloumcount = gvDetails.Rows[0].Cells.Count;
gvDetails.Rows[0].Cells.Clear();
gvDetails.Rows[0].Cells.Add(new TableCell());
gvDetails.Rows[0].Cells[0].ColumnSpan = coloumcount;
gvDetails.Rows[0].Cells[0].Text = "No Record Found";


}
}

protected void gvDetails_RowCancelingEdit(object sender, GridViewCancelEditEventArgs e)
{

}
protected void gvDetails_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName.Equals("Add New"))

{
TextBox txtUsrName = (TextBox)gvDetails.FooterRow.FindControl("txtUsrName");
TextBox txtCity = (TextBox)gvDetails.FooterRow.FindControl("txtCity");
TextBox txtdesignation = (TextBox)gvDetails.FooterRow.FindControl("txtdesignation");
con.Open();
SqlCommand cmd = new SqlCommand("Insert into Employees_Details(UserName,City,Designation) values('"+txtUsrName.Text+"','"+txtCity.Text+"','"+txtdesignation.Text+"')",con);
int result = cmd.ExecuteNonQuery();
con.Close();
if (result == 1)
{
BindEmployeeDetails();
LblResult.ForeColor = Color.Green;
LblResult.Text = txtUsrName.Text + "Details Inserted Successfully";

}

else
{

LblResult.ForeColor = Color.Red;
LblResult.Text = txtUsrName.Text + "Details Not Inserted";

}


}
}
protected void gvDetails_RowDeleted(object sender, GridViewDeletedEventArgs e)
{

}
protected void gvDetails_RowDeleting(object sender, GridViewDeleteEventArgs e)
{
int UserId = Convert.ToInt32(gvDetails.DataKeys[e.RowIndex].Values["UserId"].ToString());
string UserName = gvDetails.DataKeys[e.RowIndex].Values["UserName"].ToString();
con.Open();
SqlCommand cmd = new SqlCommand("Delete From Employees_Details where UserId=+UserId", con);
con.Close();
if (result == 1)
{
BindEmployeeDetails();
LblResult.ForeColor = Color.Red;
LblResult.Text = UserName + "Details Deleted Successfully";


}

}
protected void gvDetails_RowEditing(object sender, GridViewEditEventArgs e)
{
gvDetails.EditIndex = e.NewEditIndex;
BindEmployeeDetails();

}
protected void gvDetails_RowUpdated(object sender, GridViewUpdatedEventArgs e)
{

}
protected void gvDetails_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
int UserId = Convert.ToInt32(gvDetails.DataKeys[e.RowIndex].Value.ToString());
string UserName = gvDetails.DataKeys[e.RowIndex].Values["UserName"].ToString();
TextBox txtcity = (TextBox)gvDet