Button tooltip fails with "this.btnEl is undefined" in 3.0.3

[crashedsnow]

Apologies if this is part of the migration instructions that I have missed. I'm trying to upgrade an existing 2.2.1 app to 3.0.3, and am having a problem with a fairly simple area.

I have a Toolbar on a Ext.tree.TreePanel. The toolbar has a button. The button has a tooltip.



var btnRefresh = new Ext.Toolbar.Button (
{
id:'treeRefresh',
text: '',
iconCls: 'x-btn-text x-tbar-loading',
tooltip:'Refresh',
scope: this,
}
);

var toolbar = new Ext.Toolbar(
{
items: [btnRefresh]
}
);

tree = new Ext.tree.TreePanel(
{
id: 'objFileTree',
title:'Folders',
...
bbar: toolbar,
...



When the tree renders, I get the following failure:

this.btnEl is undefined

Reported at:

ext-all-debug.js
Line 42006 (3.0.3)

The line in question is:



this.btnEl.dom[this.tooltipType] = tooltip;


If I look at the stack in debug (Firebug), I can see that the "this" pointer points to the button, but the object does not have a "btnEl" field. It does have an "el" field however.

I've obviously missed something in the migration as this would appear to be a fairly basic feature.

I have:

Included the new CSS
Included the new images

What am I doing wrong here?