MixedCollection中添加组件出错

[北方的狼]

var viewport = new Ext.Viewport({
layout:'border',
listeners: {
beforerender: function() {
var p1 = new Ext.Panel({"title":"资产台帐","html":"aaaaaaa"});
var p2 = {"title":"资产台帐","html":"aaaaaaa","xtype":'panel'};
var item = new Ext.util.MixedCollection();
item.add('a', p1);
item.add('b', p2);
Ext.getCmp("west-panel").items = item;
},
items:[
{
region:'west',
id:'west-panel',
title:'导航',
split:true,
width: 200,
minSize: 175,
maxSize: 400,
collapsible: true,
margins:'0 0 0 5',
layout:'accordion',
layoutConfig:{
animate:true
}
},{
region:'center',
id:'mainPanel',
border:false
}
]
});
代码中如果使用代码item.add('b', p2);则报错D.render is not a function，不知道为什么，是不是延时渲染不能在这里使用呀。请大家帮忙看看