เจอ Error Object reference not set to an instance of an object. เเก้ยังไงครับ

  panelad01.Visible = false;
            con.Close();
            con.Open();
                string chkrank = "Select Status_ID from [member] where UserName='" + Session["user"]+ "'";
                SqlCommand rankid = new SqlCommand(chkrank, con);
                string rankuser = rankid.ExecuteScalar().ToString();
                if (rankuser == "1")
                {
                    Response.Write("คุณไม่ใช่ Admin");
                    Response.Redirect("/Login.aspx");
                }
                else  if (rankuser == "2")
                {
                    //
                }
                else
                {
                    Response.Redirect("admindefault.aspx");
                }