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.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Xml.Linq;

using System.Data.SqlClient;
using System.IO;
namespace EDS
{
    public partial class WebForm4 : System.Web.UI.Page
    {
        String strcon = ConfigurationManager.ConnectionStrings["con"].ToString();
        SqlConnection Scon;

        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                BindGridviewData();
            }
        }
  private void BindGridviewData()
{
    Scon = new SqlConnection(strcon);

Scon.Open();
SqlCommand cmd = new SqlCommand("select * from SaveData",Scon);
SqlDataAdapter da = new SqlDataAdapter(cmd);
DataSet ds = new DataSet();
da.Fill(ds);
Scon.Close();
gvDetails.DataSource = ds;
gvDetails.DataBind();
}
   protected void lnkDownload_Click(object sender, EventArgs e)
{
LinkButton lnkbtn = sender as LinkButton;
GridViewRow gvrow = lnkbtn.NamingContainer as GridViewRow;
string filePath = gvDetails.DataKeys[gvrow.RowIndex].Value.ToString();
Response.ContentType = "image/jpg";
Response.AddHeader("Content-Disposition", "attachment;filename=\"" + filePath + "\"");
Response.TransmitFile(Server.MapPath(filePath));
Response.End();
}

   protected void lnkdelete_Click(object sender, EventArgs e)
   {

       LinkButton lnkbtn = sender as LinkButton;
       //getting particular row linkbutton
       GridViewRow gvrow = lnkbtn.NamingContainer as GridViewRow;
       //getting userid of particular row
       string filePath = gvDetails.DataKeys[gvrow.RowIndex].Value.ToString();
       
       //string username = gvrow.Cells[0].Text;
       Scon.Open();
       SqlCommand cmd = new SqlCommand("delete from SaveData where FilePath=" + filePath, Scon);
       int result = cmd.ExecuteNonQuery();
       Scon.Close();
       if (result == 1)
       {
           BindGridviewData();
           //Displaying alert message after successfully deletion of user
           ScriptManager.RegisterStartupScript(this, this.GetType(), "alertmessage", "javascript:alert('" + filePath + " details deleted successfully')", true);
       }