System.NullReferenceException

when i open Response.Redirect("~/booking_confirm.aspx") then there arise a NullReferenceException:object reference not set to an instance   of an object, otherwise not. Why?

Please help quickly.

    protected void btn_book_Click(object sender, EventArgs e)
    {
       
        try
        {

            DataView dv44 = (DataView)SqlDataSource1.Select(DataSourceSelectArguments.Empty);
            DataTable dt6 = dv44.ToTable();
            string a = dt6.Rows[0][0].ToString();
            
            Response.Redirect("~/booking_confirm_not.aspx");
            Session["ss_booking_time"] = lbl1.Text;
            Session["ss_booking_from"] = bf.Text;
            Session["ss_booking_to"] = bt.Text;
            Session["ss_no_of_days"] = lbl_no_days.Text;
            Session["ss_total_charge"] = total_charge.Text;
            //Response.Write("booked");
        }
        catch (Exception rr)
        {
            Response.Write(rr);
           // Response.Redirect("~/booking_confirm.aspx");
            
            //Response.Write("available");
        }   
       
        
        
                      
    }