桌面模块中的extjs4“TypeError：el为null”

Question

我试图在桌面示例源上更改GridWindow.js。

Ext.define('GridWindow.GridPanel', {
    extend   : 'Ext.grid.Panel',
    alias    : 'widget.GridWindow.GridPanel',
    border: false,
    xtype: 'grid',
    store: new Ext.data.ArrayStore({
        fields: [
           { name: 'company' },
           { name: 'price', type: 'float' },
           { name: 'change', type: 'float' },
           { name: 'pctChange', type: 'float' }
        ],
        data: [
                ['3m Co',71.72,0.02,0.03],
                ['Alcoa Inc',29.01,0.42,1.47],
                ['American Express Company',52.55,0.01,0.02],
                ['American International Group, Inc.',64.13,0.31,0.49],
                ['AT&T Inc.',31.61,-0.48,-1.54],
                ['Caterpillar Inc.',67.27,0.92,1.39],
                ['Citigroup, Inc.',49.37,0.02,0.04],
                ['Exxon Mobil Corp',68.1,-0.43,-0.64],
                ['General Electric Company',34.14,-0.08,-0.23],
                ['General Motors Corporation',30.27,1.09,3.74],
                ['Hewlett-Packard Co.',36.53,-0.03,-0.08],
                ['Honeywell Intl Inc',38.77,0.05,0.13],
                ['Intel Corporation',19.88,0.31,1.58],
                ['Johnson & Johnson',64.72,0.06,0.09],
                ['Merck & Co., Inc.',40.96,0.41,1.01],
                ['Microsoft Corporation',25.84,0.14,0.54],
                ['The Coca-Cola Company',45.07,0.26,0.58],
                ['The Procter & Gamble Company',61.91,0.01,0.02],
                ['Wal-Mart Stores, Inc.',45.45,0.73,1.63],
                ['Walt Disney Company (The) (Holding Company)',29.89,0.24,0.81]
               ]
    }),
    columns: [
        new Ext.grid.RowNumberer(),
        {
            text: "Company",
            flex: 1,
            sortable: true,
            dataIndex: 'company'
        },
        {
            text: "Price",
            width: 70,
            sortable: true,
            renderer: Ext.util.Format.usMoney,
            dataIndex: 'price'
        },
        {
            text: "Change",
            width: 70,
            sortable: true,
            dataIndex: 'change'
        },
        {
            text: "% Change",
            width: 70,
            sortable: true,
            dataIndex: 'pctChange'
        }
    ]
});

Ext.define('MyDesktop.GridWindow', {
    extend: 'Ext.ux.desktop.Module',

    requires: [
        'Ext.data.ArrayStore',
        'Ext.util.Format',
        'Ext.grid.Panel',
        'Ext.grid.RowNumberer'
    ],

    id:'grid-win',

    init : function(){
        this.launcher = {
            text: 'Grid Window',
            iconCls:'icon-grid'
        };
    },

    createWindow : function(){
        var desktop = this.app.getDesktop();
        var win = desktop.getWindow('grid-win');
        if(!win){
            win = desktop.createWindow({
                id: 'grid-win',
                title:'Grid Window',
                width:740,
                height:480,
                iconCls: 'icon-grid',
                animCollapse:false,
                constrainHeader:true,
                layout: 'fit',
                items: [
                    {
                        xtype: 'GridWindow.GridPanel'
                    }
                ],
                tbar:[{
                    text:'Add Something',
                    tooltip:'Add a new row',
                    iconCls:'add'
                }, '-', {
                    text:'Options',
                    tooltip:'Modify options',
                    iconCls:'option'
                },'-',{
                    text:'Remove Something',
                    tooltip:'Remove the selected item',
                    iconCls:'remove'
                }]
            });
        }
        return win;
    }
});

我可以打开GridWindow模块并关闭。关闭窗口后，我再也无法打开GridWindow模块。

因为“new Ext.grid.RowNumberer（）”

为什么？