Ext.ux.IFrame: Uncaught TypeError

How to replicate:

Using ExtJS 5.x default app, use a couple of Ext.ux.IFrame in the tab panel.



Error:

Uncaught TypeError: Cannot call method 'on' of undefined IFrame.js?_dc=1396726800639:128



Ext.define.onLoadIFrame.js?_dc=1396726800639:128

Ext.define.doFireController.js?_dc=1396726800639:244

Ext.define.fireController.js?_dc=1396726800639:122

Ext.define.dispatchDirectEventDispatcher.js?_dc=1396726800639:494

Ext.define.onDirectEventDom.js?_dc=1396726800639:662