Why does this not work?

Code:
var cockpit = Ext.getCmp('cockpit'); // This is a Panel in a TabPanel
var sidebar = Ext.create('Ext.panel.Panel',{
    region: 'east',
    width: '200px',
    html: '<h1>TEST</h1>',
    collapsible: true
});
cockpit.add(sidebar);
I see the new Item in Firebug as second item in region 'east' (after 'center'), but it will not be rendered.

What am I doing wrong?