Работа с сессией из другого потока

  public partial class WebForm1 : System.Web.UI.Page
    {        
        protected void Page_Load(object sender, EventArgs e)
        {
            Session["test"] = 33;

            Typed.f_ClientID = 55;            

            ThreadStart ts = new ThreadStart(Treat);
            Thread thread = new Thread(ts);
            thread.Start();

            Session["test"] = 88;            
        }

        void Treat()
        {
            try
            {
                int i = (int)Session["test"];
                object obj = Typed.f_ClientID;              
            }
            catch (Exception exc)
            {

            }
        }
    }

   
    public static class Typed
    {   

        public static object f_ClientID
        {
            get { object value; m_SessionGet("ClientID", out value); return value; }
            set { m_SessionSet("ClientID", value); }
        }       
               
        private static void m_SessionGet<T>(String varName, out T value)
        {
            value = (T)HttpContext.Current.Session[varName];
        }       
               
        private static void m_SessionSet<T>(String varName, T value)
        {
            HttpContext.Current.Session[varName] = value;            
        }
    }

...
            catch (Exception exc)
            {

            }
...

...
            catch (Exception exc)
            {

            }
...

{System.NullReferenceException: Object reference not set to an instance of an object.

    private static HttpContext _ctx;

    public static void SetContext(HttpContext ctx) { _ctx = ctx; }

    public static string ClientID
    {
        get { string value; GetValue("ClientID", out value); return value; }
        set { SetValue("ClientID", value); }
    }

    private static void SetValue<T>(string key, T value)
    {
        _ctx.Session[key] = value;
    }

    private static void GetValue<T>(string key, out T value)
    {
        value = (T)_ctx.Session[key];
    }