PostBackUrl troubles

Hi: 

I get an (object reference not set to an instance of an object) error when I use a PostBackUrl as a method to pass data between pages.  Please see following code.  The code in red is highlighted in the error.  This error occurs infrequently and seemingly randomly.


page 1------

<






codebehind  for page 2------

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


public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
string previouslogin;
previouslogin = ((TextBox)PreviousPage.FindControl("logintxt")).Text;
.
.
.
.
.
.
}
}
}