使用dojo控件 Tree时，tree为空

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Product List</title>
<script type="text/javascript" src="/lib/dojo/dojo.js" djConfig="parseOnLoad:true"></script>
<script type="text/javascript" src="lib/dojo/data/ItemFileReadStore.js"></script>
<script type="text/javascript" src="lib/dijit/Tree.js"></script>
<script>
dojo.require("dojo.data.ItemFileReadStore");
dojo.require("dijit.Tree");
</script>
<script type="text/javascript" src="lib/dijit/tests/_testCommon.js"></script>
</head>
<body>
<div dojoType="dojo.data.ItemFileReadStore" jsId="continentStore" id="productCatalog"
url="/lib/dijit/tests/_data/countries.json" title="/lib/dijit/tests/_data/countries.json"></div>
<div dojoType="dijit.tree.ForestStoreModel" jsId="continentModel" 
store="continentStore" query="{type:'continent'}"
rootId="continentRoot" rootLabel="Continents" childrenAttrs="children" openOnClick="true" event="onclick">
</div>
<div dojoType="dijit.Tree" id="mytree"
model="continentModel" openOnClick="true">
<script type="dojo/method" event="onClick" args="item">
var tree = dijit.byId("mytree");
alert(tree == null);
var tdata=[{label:"name",item:{type:"city",name:"Paris"}}];
var _treeNode = new dijit._TreeNode(tdata);
var fileReadData = new dojo.data.ItemFileReadStore({data:tdata});
</script>
</div>

</body>
</html>在Firefox下调试时总是提示tree is undefined
[Break on this error] var tree = this.tree, model = tree.model;当alert（tree）；时tree不为空且是Widget dijit.Tree, mytree，请问下是什么原因。dojo的版本是最新的
请高手多多指教