Object reference not set to an instance of an object.

My Code is :
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)
    {
        gvDetails.EditIndex = -1;
        BindEmployeeDetails();
    }
    protected void gvDetails_RowCommand(object sender, GridViewCommandEventArgs e)
    {
        if (e.CommandName.Equals("AddNew"))
        {
            TextBox txtUserName = (TextBox)gvDetails.FooterRow.FindControl("txtUserName");
            TextBox txtCity = (TextBox)gvDetails.FooterRow.FindControl("txtCity");
            TextBox txtdesignation = (TextBox)gvDetails.FooterRow.FindControl("txtdesignation");
            con.Open();
            if (txtCity.Text == "" || txtdesignation.Text == "" || txtUserName.Text == "")
            {
   // Response.Write(@"<Script language='JavaScript'> alert('Enter The Details Properly');void ('');</script">);

                string textForMessage = @"<script language='javascript'> alert('Enter The Complete Details');</script>";

    Page.ClientScript.RegisterClientScriptBlock(this.GetType(), "UserPopup", textForMessage);
            }

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

                }

                else
                {

                    LblResult.ForeColor = Color.Red;
                    LblResult.Text = txtUserName.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)gvDetails.Rows[e.RowIndex].FindControl("txtcity");
        TextBox txtDesignation = (TextBox)gvDetails.Rows[e.RowIndex].FindControl("txtDesigantion");
        con.Open();
        SqlCommand cmd = new SqlCommand("update Employees_Details set City='" + txtcity.Text + "',Designation='" + txtDesignation.Text + "' where UserId=" + UserId, con);
        cmd.ExecuteNonQuery();
        con.Close();
        LblResult.ForeColor = Color.Green;
        LblResult.Text = UserName + "Details Updated Successfully";
        gvDetails.EditIndex = -1;
        BindEmployeeDetails();
    }
    protected void gvDetails_RowCreated(object sender, GridViewRowEventArgs e)
    {

    }
    protected void gvDetails_SelectedIndexChanged(object sender, EventArgs e)
    {

    }
    protected void gvDetails_SelectedIndexChanging(object sender, GridViewSelectEventArgs e)
    {

    }
}