We are not quite sure that we understood your scenario. You could load pages that contains Master pages or nested master pages inside a RadWindow
without any problems and we are not sure what is causing the error in your case.
To be able to help, in this case, we need to examine your exact setup. Could you please open an new support ticket and send us a full sample project which reproduces the problem that you experience? Once we have a better view over your scenario we will do our best to provide a solution.
All the best,
the Telerik team