Thank you for taking the time to investigate this. I am logging it for further research on our end and I have also updated your Telerik points for sharing this information with us.
Generally, the getWindowBounds() method is designed to work on an already shown RadWindow, hence the lack of such a check. You can emulate it by using the isVisible() method as a flag:
wnd = $find(
"<%= RadWindow1.ClientID %>"
(!wnd.isVisible()) wnd.show(); //or simply return
the Telerik team
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