ExtJS 4.1.3
Chrome Macintosh

On line 185 of src/panel/Header.js in afterLayout

titleEl = me.titleCmp.el;

I occasionally get an exception where titleCmp is undefined. This happens when me is a Bar (src/tab/Bar.js). Bar is a subclass of Header. You'll notice in Bar's initComponent it deletes titleCmp:

delete me.titleCmp;

So it will fail for Bar when it calls the parent's afterLayout. I suspect you should check for titleCmp in Header's afterLayout to fix the bug