How to fix "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;
using System.Web.UI.WebControls;
using System.Data;
using System.Data.SqlClient;

public partial class home : System.Web.UI.Page
{
    SqlConnection con;
    SqlCommand cmd;
    SqlDataAdapter adp;
    DataSet ds;

    void connect()
    {
        con = new SqlConnection(@"Data Source=.\SQLEXPRESS;AttachDbFilename=C:\Users\Abhik\Documents\Visual Studio 2010\WebSites\XP\App_Data\Database.mdf;Integrated Security=True;User Instance=True");
        con.Open();
    }

    protected void Page_Load(object sender, EventArgs e)
    {
        connect();
        adp = new SqlDataAdapter("select * from user_info where u_mail='"+Session["id"].ToString()+"'", con);
        ds = new DataSet();
        adp.Fill(ds);
        Label1.Text = ds.Tables[0].Rows[0][0].ToString();
        Label2.Text = ds.Tables[0].Rows[0][1].ToString();
        Label3.Text = ds.Tables[0].Rows[0][3].ToString();
        Label4.Text = ds.Tables[0].Rows[0][5].ToString();
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        Response.Redirect("signin.aspx");
    }
}

Line 26: adp = new SqlDataAdapter("select * from user_info where u_mail='"+Session["id"].ToString()+"'", con);