Masterpage and contentpage session variable accessing problems in C#.net

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ASP.NET

hello please answer me
i have used a master page in which i have design a login form for administrator i used the session variable on master page when admin give right login name and password it mean i store the login name in session["admin"] variable then i access this variable in content page onload() event but when i check this variable a run time exception occur
(Object reference not set to an instance of an object.)
this is my onload() event of the content page

protected void Page_Load(object sender, EventArgs e)
   {
       //TextBox admin = Master.FindControl("txtAdmin") as TextBox;
       //TextBox pass = Master.FindControl("txtPassword") as TextBox;
       //Button log = Master.FindControl("btnLogin") as Button;
       this.Title = "NU-Computer Science";
       if (IsPostBack)
       {
           if (Session["admin"].ToString().Trim() != "" || Session["admin"] != null)
           {
               pnlAdmin.Visible = true;
           }
       }
           FillRepeater();

   }

and this is my master page code

protected void btnLogin_Click(object sender, EventArgs e)
    {
        Admin admin = new Admin();  // this is a class where i check the user name and password
        admin.UserName = this.txtAdmin.Text;  // set the user name property
        admin.Password = this.txtPassword.Text; // set the password property
        string user = admin.adminName();  // adminName() is a function which return user name
        if ((user.Length == 0 || user == ""))
        {
            lblError.ForeColor = System.Drawing.Color.Red;
            lblError.Text = "Wrong user or password";
        }
        else
        {
            Session["admin"] = txtAdmin.Text;
        }
    }

Please help me in this Issue
thanks in Advance
give me answer here or sent to my email <email deleted to limit spam>

Note: I know that the content pageload() event rise before the master pageload event i sure that this is the problem but tell me that what i do? with this essue