The controls in the RadControls for ASP.NET AJAX suite are built upon the MS AJAX framework and the framework itself creates its controls (including RadWindowManager) in the Sys.Application.add_init() method.
Here is how the client events are fired in the page life cycle:
window.onload -> Sys.Application.init -> Sys.Application.load
You can see that the client object representation of MS AJAX controls are created just as the last code on the page in the Sys.Application.init event, which is raised after the window.onload event.
That is why when you call your code in window.onload by registering the client script, the RadWindowManager will still not be created on the page and you will get an error. To avoid this problem, you can either execute your code with a small timeout, or use the Sys.Application.add_load method.
You can find additional information about this here:
That is why what you could do in this case is to choose one of the following options:
1) Set a bigger timeout which ensures that the manager is already created
2) Use the Sys.Application.add_load event as explained above
I hope that the provided explanation and resource are helpful, let me know how it goes.
the Telerik team
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