Hello Experts,
I have a page with the GridView control on it. When I select the record inside the grid, the control goes to the "details" page and show the result correctly. But, if I click on the "Back" button in the browser and select another record from the grid, I am getting an error "Object reference not set to an instance of an object"
at the selected line of code shown below.

protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
        {
            XmlDocument usersList = null;
            long usersNumber = 0;
            int index;

            try
            {
                Session["selectedName"] = GridView1.SelectedDataKey.Values["Name"].ToString();

                if (Session["selectedName"].ToString() != "")
                {
                    try
                    {
                        //Create the proxy.
                        ADUsersService proxy = new ADUsersService();
                        //Call the web service and get the results:
                        XmlNode myXMLNodeObject = proxy.getUserAccount(Session["selectedName"].ToString());
                        usersList = new XmlDocument();
                        usersList.AppendChild(usersList.ImportNode(myXMLNo deObject, true));

                        if (usersList != null)
                        {
                            Session["usersList"] = usersList;
                            usersNumber = proxy.SearchResultCount();
                            if (usersNumber != 0)
                            {
                                if (usersNumber != 1)
                                {
                                    //Redirect to the Multy Result page
                                    Response.Redirect("MultyResult.aspx", false);
                                }
                                else
                                {
                                    //Redirect to the Single Result page
                                    Response.Redirect("SingleResult.aspx", false);
                                }
                            }
                            else
                            {
                                //Redirect to the Message page
                                Session["appMessage"] = "No matching user records found or selected record's Display User Name field is empty. Try different search options, please.";
                                //Redirect to the Message page
                                Response.Redirect("MessageForm.aspx", false);
                            }
                        }
                        else
                        {
                            //Redirect to the Message page
                            Session["appMessage"] = "No matching user records found. Try different search options, please.";
                            //Redirect to the Message page
                            Response.Redirect("MessageForm.aspx", false);
                        }
                        proxy.Dispose();
                    }
                    catch (Exception ex)
                    {
                        Session["errorMessage"] = ex.Message.ToString();
                        //Redirect to the Error page
                        Response.Redirect("ErrorForm.aspx", false);
                    }
                }
                else
                {
                    //Redirect to the Message page
                    Session["appMessage"] = "No matching user records found. Try different search options, please.";
                    //Redirect to the Message page
                    Response.Redirect("MessageForm.aspx", false);
                }
                //Redirect to the Single Result page
                Response.Redirect("SingleResult.aspx", false);
            }
            catch (Exception ex)
            {
                Session["errorMessage"] = ex.Message.ToString();
                //Redirect to the Error page
                Response.Redirect("ErrorForm.aspx", false);
            }

        }

It looks like GridView1.SelectedDataKey value == null (disappeared) by some reason?

What should I do to solve this issue?

Thanks in advance for reading this,
Dmitriy