extjs - vbox with children

HTML

<link rel="stylesheet" type="text/css" href="//cdn.sencha.io/ext-4.1.1-gpl/resources/css/ext-all-debug.css" />

JavaScript

Ext.define('WD40.view.legend.Utils', {
    singleton: true,

    createTreeColumsSpec: function(renderFunction) {
        return [{
            xtype: 'treecolumn',
            flex: 1,
            text: 'text',
            dataIndex: 'text',
            renderer: renderFunction
        }];
    },
    createTools: function(prefix) {
        return [{
            type: 'plus',
            id: prefix + 'plustool',
            tooltip: "Show selected document highlights"
        }, {
            type: 'minus',
            id: prefix + 'minustool',
            tooltip: "Hide selected document highlights"
        }];
    }
});

Ext.define('WD40.view.Legend', {
    extend: 'Ext.panel.Panel',
    alias: 'widget.wd40legend',
    requires: ['WD40.view.legend.Utils'],

    layout: {
			    type: 'vbox',
			    align : 'stretch'
			},
    items: [
        {
        xtype: 'treepanel',
        title: 'Base Linguistics',
        width: '20%',
        border: 0,
        rootVisible: false,
        itemId: 'bltree',
        id: 'bltree',
        useArrows: true,
        collapsible: true,
        collapseFirst: false,
        collapsed: false,
        hideHeaders: true,
        root: {
            text: 'Root',
            expanded: true,
            children: [{
                text: 'hello1',
                expanded: true,
                children: [
                    {
                    text: 'child 1',
                    leaf: true},
                {
                    text: 'child 2',
                    leaf: true},
                {
                    text: 'child 3',
                    leaf: true},
                {
                    text: 'child 4',
                    leaf: true},
                {
                    text: 'child 5',
                    leaf: true},
                {
                    text: 'child 6',
                    leaf: true},
                {
                    text: 'child 7',
                    leaf: true},
               ...