We have changed the implementation in this manner in order to provide ViewState support for the Windows collection because there was not such. With the version you are using in order to achieve what you need you can indeed iterate through the windows in the Windows collection to find your window or e.g save its index in the Session object and get it by index.
In the Beta release, however, the RadWindows are already added to the Controls collection of the RadWindowManager and thus if you upgrade to it, you can directly reference the RadWindow by using the FindControl method as shown below:
var wlogin = WMPagina.FindControl(
The FindControl method solution will work with every later release than the Beta as well.
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