help me out! Object reference not set to an instance of an object

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

using System.Web.UI.WebControls;
using System.Data.SqlClient;
using System.Data;

namespace Datile
{
    public partial class login : System.Web.UI.Page
    {
     
      public  static string s = System.Configuration.ConfigurationManager.ConnectionStrings["ApplicationDbContext"].ToString();
        SqlConnection con = new SqlConnection(s);
        protected void Page_Load(object sender, EventArgs e)
        {

        }
        protected void Login1_Authenticate(object sender, AuthenticateEventArgs e)
        {
        }
        protected void Button1_Click(object sender, EventArgs e)
        {
            if ((TextUname.Text != "") && (TextPassword.Text != ""))
            {
                con.Open();

                SqlCommand cmd = new SqlCommand("select * from login where username='" + TextUname.Text + "'", con);
                SqlDataReader dr = cmd.ExecuteReader();
                if (dr.Read())
                {
                    if (TextUname.Text == dr["username"].ToString())
                    {
                        if (TextPassword.Text == dr["password"].ToString())
                        {

                            Session["name"] = dr["username"].ToString();
                            Session["uid"] = dr["uid"].ToString();
                         

                        }
                        else
                        {

                            // if (TextBox2.Text != dr["pwd"].ToString())
                            Label3.Text = "Incorrect Password";
                        }
                    }
                    else if (TextUname.Text != dr["username"].ToString() && TextPassword.Text != dr["password"].ToString())
                    {
                        Label3.Text = "both UserName and Password are wrong";
                    }
                    else
                    {
                        Label3.Text = "Incorrect UserName ";
                    }
                }
                else
                {
                    Label1.Text = "both UserName and Password are wrong";
                }
                dr.Close();
                con.Close();
            }
        }

    }
}