Extjs Ext.data.Store使用有关问题

Extjs Ext.data.Store使用问题
大家好，我最近在使用Extjs模板，遇到个问题，想请教大家，我在用Ext.data.Store读取服务器的数据，在前台显示的时候显示不出东西来，这是怎么回事。
以下是我的后台代码：

PHP code

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    function actionReadAllDep()
    {
        $pageLimit = getPageLimit();
        $erp = new ModelErp();
        $sql = "SELECT * FROM es_dep_pro";
        $dep = $erp->getList($sql,$pageLimit['form'],$pageLimit['to']);
        $result['totalCount'] = count($dep);
        $result['topics'] = $dep;
        require(CFG_PATH_LIB.'util/JSON.php');
        $json = new Services_JSON();
        echo $json->encode($result);
    }

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JScript code

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    Ext.onReady(function(){
        var ds = new Ext.data.Store({
            proxy : new Ext.data.HttpProxy({url:'index.php?model=erp&action=ReadAllDep&'}),
            reader: new Ext.data.JsonReader({
                root: 'topics',
                totalProperty: 'totalCount'
            },[
                'DEP_ID','DEP_NAME','UPPER_ID','STATUS'
            ])
        });
        var cm = new Ext.grid.ColumnModel([
            {header:"部门编号",width:80,dataIndex:"DEP_ID"},
            {header:"部门名称",width:100,dataIndex:"DEP_NAME"},
            {header:"上级部门编号",width:80,dataIndex:"UPPER_ID"},
            {header:"部门状态",width:30,dataIndex:"STATUS"}
        ]);
        cm.defaultSortable = true;
        /*var grid = new Ext.grid.GridPanel({
            region:'center',
            loadMask:true,
            store:ds,
            cm:cm,
            stripeRows:true,
            border:true,
            borderStyle:'width:100',
            autoScroll:true
        });*/
        ds.load({params: {start: 0,limit: 20}});
        loadend();
    });

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