The function GetRadWindowManager returns NULL in IE 11 (and probably 10). Both, Chrome and Firefox working fine. The Function is executed manually by clicking a button, so I'm sure the page is completely loaded.
I'm working with Telerik 2013.2 SP1. Is this fixed in current releases?
There are no known issues between IE11 and Q2 2013 SP1, yet official support for this browser was introduced in the Q3 2013 SP1 release (2013.3.1114) and I advise that you upgrade to that version.
I tried a simple page with that logic with both version and things seemed to work fine for me. You can find attached the page and a short video as a reference.
In the meantime you can go with the $find(wndMngrClientID) to get a proper reference. You can create a function next to the control that will return the needed reference and use that instead of GetRadWindowManager() to see if this helps.
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