Programmer - session variable object gets deleted

Programmer Question

I have made something like the following code:

protected void Page_Load(object sender, EventArgs e)
{
   Label1.Text = Session["loginid"].ToString();
}

protected void delete_click(object sender, EventArgs e)
{
    delete("mail1",Session["loginid"]);
}

private int delete(string mailid, string user)
{
 System.IO.Directory.Delete(Server.MapPath(@"~\files\" + user + @"\" + mailid), true);
}

When i press the delete button, everything works fine and the folder gets delted.
but after that when page postbacks again then a
NullRefrenceException is raised at
Label1.Text = Session["loginid"].ToString();

why is it happening...??
Please help.