dynamic add items to border viewport

I want to create a empty viewport(layout:border) first, then add items to it, finally render and show. but when I created the viewport element, it throw an error, says:

Uncaught TypeError: Cannot read property 'flex' of undefined

here are my code:

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

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

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

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

According to notes 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 have a child item with region:'center'