Object reference not set to an instance of an object.

Object reference not set to an instance of an object.

未將對象引用設置到對象的實例

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

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

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

aspx页：

  <ext:Tree ID="Tree1" runat="server" AutoScroll="true" EnableArrows="true" Title="Tree">
    </ext:Tree>

 <asp:XmlDataSource ID="XmlDataSource1" runat="server" DataFile="~/menu.xml">
    </asp:XmlDataSource>

cs页面：

   DataSet ds = new DataSet();
            string path = Server.MapPath("menu.xml");
            ds.ReadXml(path);
            Tree1.DataSource = ds;
            Tree1.DataBind();

  //Tree1.DataSource = ds;
            //Tree1.DataSource = XmlDataSource1;

求解？？？？