When TabPanel items is empty items gets set to items: [ null ], causing a null to be passed to isValidParent in Ext.layout.container.AbstractCard.

Error in FireBug:

item is null
[Break On This Error] var itemEl = item.el ? item.el.dom : Ext.getDom(item);

To work around this bug, put something in the tabpanel to begin with. If you remove all of the tabs from the tabpanel you'll get the same error.