If the context page (newpopup.aspx) comes from a different domain, the error is expected as you cannot access objects from the parent page from within the parent page. The issue is not related to the RadWindow control - you can try getting an access to another JS object in the parent and you will receive the same access denied message. To avoid that, you could try to use document.domain to set the domain property of the document object to the domain of the other page.
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