It seems you are still using the client-side API for the "classic" RadMenu.
RadMenu for ASP.NET AJAX has different client-side API and you can check these articles on the subject:
In your case, you need to replace this line:
var linkElement = eventArgs.Item.LinkElement;
with this one:
var linkElement = eventArgs.get_item().get_linkElement();
I hope this helps.
the Telerik team
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