"Error: Object reference not set to an instance of an object.(NullReferenceException was unhandled by user code)"

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"Error: Object reference not set to an instance of an object.(NullReferenceException was unhandled by user code)"

Can you please suggest me to come out of the error !.

I was created registration page, and also created database. when we submit that registration fields that fields should be stored in database(my database name is "registration"). i think it's connection strings problem.I written a code in "Web.config" for connection string.



Can you please suggest me to come out of the error !.





CODE:

using System;
using System.Configuration;
using System.Data;
using System.Collections.Generic;
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.Xml.Linq;
using System.Data.SqlClient;


public partial class Default2 : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
    SqlConnection con = new SqlConnection("Data Source=TEKPC26\\SQLEXPRESS;Initial Catalog=registration;Integrated Security=True");
    SqlCommand cmd;
    protected void Button1_Click(object sender, EventArgs e)
    {

        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["registration"].ToString());
        con.Open();
        cmd=new SqlCommand("insert into values('" + TextBox1.Text + "','" + TextBox2.Text + "','" + TextBox3.Text+"')",con);
        cmd.ExecuteNonQuery();

    }
}

AND

Web.config CODE:

<configuration>

  <connectionStrings>

        <add name="ApplicationServices" connectionString="data source=.TEKPC26\\SQLEXPRESS;
         Integrated Security=SSPI;AttachDBFilename=C:\\Documents and Settings\\Administrator\\My Documents\\Visual Studio 2010\\WebSites\\WebSite1\\App_Data\\registration.mdf;
         User Instance=true" providerName="System.Data.SqlClient"/>
    </connectionStrings>