extaspnet框架的问题

页面如下
<body>
    <form id="form1" runat="server">
    <div>    
             
        <ext:PageManager ID="PageManager1" runat="server" />
              <ext:Window ID="Window1" runat="server" 
             Title="系统后台登陆" 
             IsModal="false" EnableClose="false"
            Width="350px">
            <Items>        
                <ext:SimpleForm ID="SimpleForm1" 
                runat="server" ShowBorder="false" 
                BodyPadding="10px"
                    EnableBackgroundColor="true" 
                    ShowHeader="false">
                    <Items>
                        <ext:TextBox ID="tbxUserName" Label="用户名：" 
                        Required="true" runat="server" 
                            RequiredMessage="用户名不允许为空" 
                            Text="ASPNETDemo">
                        </ext:TextBox>
                           <ext:TextBox ID="tbxPassword" 
                             Label="密&amp;nbsp;&amp;nbsp;&amp;nbsp;码：" 
                            TextMode="Password" Required="true"
                            runat="server" RequiredMessage="密码不允许为空" Text="888888">
                        </ext:TextBox>
                        <ext:TextBox ID="tbxCaptcha" Label="验证码：" 
                            Required="true" runat="server" 
                            RequiredMessage="验证码不允许为空">
                        </ext:TextBox>
                                                      
                    </Items>
                </ext:SimpleForm>
            </Items>
        </ext:Window>
    </div>
    </form>
</body>
然后运行下，就老是提示：未将对象引用设置到对象的实例。
[NullReferenceException: 未将对象引用设置到对象的实例。]
   ExtAspNet.PageManager.get_EnableAjax() +88
   ExtAspNet.PageManager.OnFirstPreRender() +1130
   ExtAspNet.ControlBase.OnPreRender(EventArgs e) +231
   System.Web.UI.Control.PreRenderRecursiveInternal() +103
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Control.PreRenderRecursiveInternal() +175
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +2496