Such an error may indicate that the MS AJAX framework has not been loaded at the point you try to open the RadWindow. Please confirm that the page has fully loaded when you call this function as the AJAX scripts are the last thing to be downloaded on a page. Also make sure that the RadWindow is defined in the proper page so that it includes the needed scripts and that its EnableEmbeddedScripts property is not set to false.
I also tried this approach and things seem to be working fine with me: http://screencast.com/t/dox6c9YnPKkn
. I am also attaching my test page here as a reference, so you can compare it with yours and check for any major differences that may be causing this on your end.
Please also make sure that you are using the correct ClientID of the RadWindow, especially if the case involves INmaing containers (e.g. master pages) - instead of $find("RadWindow1") use server code blocks: $find("<%=RadWindow1.ClientID %>").
Also, make sure that the frame hierarchy is correct - i.e. the frames are only one level deep, so the parent keyword will suffice. If not - you can change it to top so that you reference the topmost frame.
the Telerik team
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