int sDipl = Convert.ToInt32(GridView4.SelectedDataKey.Values[1]);

   protected void GridView3_SelectedIndexChanged(object sender, EventArgs e)
        {
            Server.Transfer("~/AddDiplom.aspx",true);
        }

 protected void Page_Load(object sender, EventArgs e)
        {
           
                this.TextBox1.Text =    
           (Page.PreviousPage.FindControl("sDipl")    
              as TextBox).Text;   
         }

{"Ошибка при выполнении запроса дочернего элемента для ~/AddDiplom.aspx."}

protected void GridView3_SelectedIndexChanged(object sender, EventArgs e)
        {

            Session["sDipl"] = Convert.ToInt32(GridView4.SelectedDataKey.Values[1]);
        }