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
}]
});
});