cannot read property 'flex' of undefined

Extjs的一个错误，谷歌浏览器捕获到的，

后来查了下原因，应该是布局的错误，你看下，你用到的border布局中，是否没有用到center布局。

下面是一个demo类似的错误会发生

http://stackoverflow.com/questions/6874016/dynamic-add-items-to-border-viewpor

<script type="text/javascript" src="ext-4.0.2a/bootstrap.js"></script>
<script>
Ext.require(['*']);
Ext.onReady(function() {
    Ext.QuickTips.init();

    var viewport = Ext.create('Ext.container.Viewport', {
        layout: 'border'
    });

    var north = blablabla...
    viewport.add([north,south]);

    viewport.doLayout();
});
</script>

According to noes that are taken from  docs :

The regions of a BorderLayout are fixed at render time and thereafter, its child Components  may not be removed or added .To add/remove Components within a BorderLayout, have them wrapped by an additional Container which is directly managed by the BorderLayout.

And by the way

Any Container using the Border layout  must