Maintain session when page opened in new tab or new window using javascript

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C#

ASP.NET

Javascript

I m using the following method to open a page in new tab for ex:- one.aspx.

ScriptManager.RegisterStartupScript(this, this.GetType(), "Open", "window.open('" + pageurl + "','_newtab','width=1300,height=660');", true);

The page loads, but when i click on a button on one.aspx which redirects me to another page for ex:- Two.aspx it gives me exception on following line of redirected page i.e... Three.aspx on page load

 protected void Page_Load(object sender, EventArgs e)
        {
              string usertype = Session["usertype"].ToString();
        }

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

why Session["usertype"] becomes null?.
Please help me out....
Thanks in advance....