asp.net的InvalidOperationException报错

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.HtmlControls;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Xml.Linq;
using System.Data.SqlClient;public partial class back_login : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        //TextBox1.Text=Session["userid"];
        if (!Page.IsPostBack)
        {
            do                SerialNumber1.Create();
            while (SerialNumber1.SN.Length != 4);
        }
       
    }
    protected void Button1_Click(object sender, EventArgs e)
    {
        if (Session["userid"] != null)
        {
            TextBox1.Text = Session["userid"].ToString();
            string uid, pwd;
            uid = TextBox1.Text;
            pwd = TextBox2.Text;            if (SerialNumber1.CheckSN(TextBox3.Text))
            {
                SqlConnection conn = new SqlConnection();
                conn.ConnectionString = ConfigurationManager.ConnectionStrings["euyar"].ToString();                try
                {
                    conn.Open();
                    SqlCommand cmd = new SqlCommand();
                    cmd.CommandText = "select zt from euyar where userid= ' " + uid + " ' and pass= '" + pwd + " '";
                    cmd.Connection = conn;
                    //int count = Convert.ToInt32(cmd.ExecuteReader());
                    SqlDataReader Permissions = cmd.ExecuteReader();
                    if (Convert.ToInt32(Permissions.GetString(0)) == 2)
                    {
                        //Session["userid"] = uid;
                        Response.Redirect("userupdata.aspx");                    }
                    else
                    {
                        RegisterStartupScript("", "<script language='javascript'>alert('用户名或密码不正确！');</script>");
                    }
                }
                catch (Exception ex)
                {
                    HttpContext.Current.Response.Write(ex.Message);
                }
                finally
                {
                    conn.Close();                }
            }
            else 
            {
                RegisterStartupScript("", "<script language='javascript'>alert('验证码错误！');</script>");
                do                    SerialNumber1.Create();
                while (SerialNumber1.SN.Length != 4);
            }        }
        else
        {
            RegisterStartupScript("", "<script language='javascript'>alert('登录超时，请重新登录！');</script>");
            Response.Redirect("login.aspx");
        }
    }
    protected void LinkButton1_Click(object sender, EventArgs e)
    {
        do            SerialNumber1.Create();
        while (SerialNumber1.SN.Length != 4);
    }
}
我运行网页后，当输入完密码和验证码后，点击登陆时，网页报错。
报错内容如下：
使用 TreeView.HoverStyle 要求 Page.Header 非空(例如 <head runat="server" />)。 
说明: 执行当前 Web 请求期间，出现未处理的异常。请检查堆栈跟踪信息，以了解有关该错误以及代码中导致错误的出处的详细信息。 异常详细信息: System.InvalidOperationException: 使用 TreeView.HoverStyle 要求 Page.Header 非空(例如 <head runat="server" />)。源错误: 执行当前 Web 请求期间生成了未处理的异常。可以使用下面的异常堆栈跟踪信息确定有关异常原因和发生位置的信息。  堆栈跟踪: 
[InvalidOperationException: 使用 TreeView.HoverStyle 要求 Page.Header 非空(例如 <head runat="server" />)。]
   System.Web.UI.WebControls.TreeView.EnsureRenderSettings() +1186759
   System.Web.UI.WebControls.TreeView.OnPreRender(EventArgs e) +59
   System.Web.UI.Control.PreRenderRecursiveInternal() +86
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Control.PreRenderRecursiveInternal() +170
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2041 
请各位大哥大姐们帮我看看，是什么错误，告诉下小弟！