我用extjs的gridpanel实现分页数据就是不显示怎么回事

json数据时这样的
{totalCount:3,success:true,error:"",singleInfo:"",data:[{"Id":"1","UerId":"1","RoleId":"1"},{"Id":"2","UerId":"2","RoleId":"2"}]}js代码
Ext.onReady(function(){
    var store = new Ext.data.Store({
        proxy:new Ext.data.HttpProxy({
url:"Default.aspx"
}),
        reader: new Ext.data.JsonReader({
            root: 'data',
            totalProperty: 'totalCount',
            id: 'threadid',
            fields:[{name:"Id",type:"int"},{name:"UerId"},{name:"RoleId"}]
        })
    });
       var cm = new Ext.grid.ColumnModel([{
header:"产品编号",
dataIndex:"Id"
},{
header:"产品类型",
dataIndex:"UerId"
},{
header:"产品名称",
dataIndex:"RoleId"
}]);      var grid = new Ext.grid.GridPanel({
        el:'topic-grid',
        width:700,
        height:500,
        title:'ExtJS.com - Browse Forums',
        ds: store,
        cm: cm,
        bbar: new Ext.PagingToolbar({
            pageSize: 8,
            store: store,
            displayInfo: true,
            displayMsg: 'Displaying topics {0} - {1} of {2}',
            emptyMsg: "No topics to display"
        })
    });
    store.load({params:{start:0,limit:8}});
    grid.render();
});加载页面如下：
<html> <head id="Head1" runat="server">
    <title></title>
<link href="resources/css/ext-all.css" rel="stylesheet" type="text/css" />
    <script src="resources/adapter/ext/ext-base.js" type="text/javascript"></script>
    <script src="resources/ext-all.js" type="text/javascript"></script>
<script src="JS/Noname2.js" type="text/javascript" charset="gb2312"></script>
</head>
<body> 
<form id="form1"  runat="server">
<div id="topic-grid"></div>
</form>
</body> 
</html>我感觉没错呀，就是显示不出来数据，那位仁兄帮忙解决下嘛，谢谢了!