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Click item on tree,the userdata is "undefined"

when I used follow code to click item on tree,the userdata's value is "undefined":
tree.attachEvent("onClick",function(id){
    var url = tree.getUserData(id,"url");
alert(url);
    window.open(url);
return true;
});

and the XML like this:

<item text="law" id="2">
<userdata name="url">http://mywebsite/mypage</userdata>
</item>

I saw almost about this problem in the kb, but I still can't solve the problem,please help,thanks!

The same code works correctly locally. 
Be sure that old xml, without userdata, is not cached