Ext 生成带有checkbox的tree选中问题

本人因为工作需要，了解了一点点的ext，利用前人的代码做了一个带有复选框的树状图，但是不知道为什么生成之后点击复选框不能选中，点击节点其他位置就能选中，求高人指点。贴出代码如下：var dataPanel;
        Ext.onReady(function () {
            // 资源列表模型定义
            Ext.define('OrgModel', {
                extend: 'Ext.data.Model',
                fields: [
                        { name: 'IndexGuid', type: 'string' },
                        { name: 'text', type: 'string' },
                        { name: 'id', type: 'string' }
                    ]
            });
            // 菜单数据源
            var store = Ext.create('Ext.data.TreeStore', {
                autoLoad: true,
                model: 'OrgModel',
                proxy: {
                    type: "ajax",
                    url: "TransactiorEd.ashx"
                },
                root: {
                    id: "",
                    text: 'Root',
                    expanded: true
                },
                listeners: {
                    //注意beforeload的参数。与3.x中的不同
                    beforeload: function (ds, opration, opt) {
                        opration.params.parentid = opration.node.data.IndexGuid; //获得节点的相应属性，也有所不同
                        opration.params.type = "unadmin";
                    }
                },
                folderSort: true
            });
            // 数据树形列表
            dataPanel = Ext.create('Ext.tree.Panel', {
                //title: '数据资源',
                width: 376,
                height: 250,
                store: store,
                autoScroll: true,
                rootVisible: false,
                useArrows: true,
                layout: 'fit',
                //renderTo: 'Div_tree',
                region: 'center',
                //             hrefTarget: 'mainFrame',
                //renderTo: Ext.getBody(),
                listeners: {
                    //选中用户
                    itemclick: function (record, item, index) {
                        if (item.data.checked != null) {
                            item.set('checked', !item.data.checked);
                        }
                        if (!item.isExpanded())
                            item.expand();
                        else {
                            item.collapse();
                        }
                    }
                }
            });