ExtJs 3.1 XmlTreeLoader Example Error

/*!
* Ext JS Library 3.1.0
* Copyright(c) 2006-2009 Ext JS, LLC
* licensing@extjs.com
* http://www.extjs.com/license
*/

//
// Extend the XmlTreeLoader to set some custom TreeNode attributes specific to our application:
//
Ext.app.BookLoader = Ext.extend(Ext.ux.tree.XmlTreeLoader, {
processAttributes : function(attr){
if(attr.first){ // is it an author node?

// Set the node text that will show in the tree since our raw data does not include a text attribute:
attr.text = attr.first + ' ' + attr.last;

// Author icon, using the gender flag to choose a specific icon:
attr.iconCls = 'author-' + attr.gender;

// Override these values for our folder nodes because we are loading all data at once. If we were
// loading each node asynchronously (the default) we would not want to do this:
attr.loaded = true;
attr.expanded = true;
}
else if(attr.title){ // is it a book node?

// Set the node text that will show in the tree since our raw data does not include a text attribute:
attr.text = attr.title + ' (' + attr.published + ')';

// Book icon:
attr.iconCls = 'book';

// Tell the tree this is a leaf node. This could also be passed as an attribute in the original XML,
// but this example demonstrates that you can control this even when you cannot dictate the format of
// the incoming source XML:
attr.leaf = true;
}
}
});

Ext.onReady(function(){

var detailsText = '<i>Select a book to see more information...</i>';

var tpl = new Ext.Template(
'<h2 class="title">{title}</h2>',
'<p><b>Published</b>: {published}</p>',
'<p><b>Synopsis</b>: {innerText}</p>',
'<p><a href="{url}" target="_blank">Purchase from Amazon</a></p>'
);
tpl.compile();

new Ext.Panel({
title: 'Reading List',
renderTo: 'tree',
layout: 'border',
width: 500,
height: 500,
items: [{
xtype: 'treepanel',
id: 'tree-panel',
region: 'center',
margins: '2 2 0 2',
autoScroll: true,
rootVisible: false,
root: new Ext.tree.AsyncTreeNode(),

// Our custom TreeLoader:
loader: new Ext.app.BookLoader({
dataUrl:'xml-tree-data.xml'
,requestMethod: 'GET'
}),

listeners: {
'render': function(tp){
tp.getSelectionModel().on('selectionchange', function(tree, node){
var el = Ext.getCmp('details-panel').body;
if(node && node.leaf){
tpl.overwrite(el, node.attributes);
}else{
el.update(detailsText);
}
})
}
}
},{
region: 'south',
title: 'Book Details',
id: 'details-panel',
autoScroll: true,
collapsible: true,
split: true,
margins: '0 2 2 2',
cmargins: '2 2 2 2',
height: 220,
html: detailsText
}]
});
});