I already answered your other thread and for your convenience and for others who need more detailed information about the AJAX client lifecycle, I pasted it below:
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 RadWindow) 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 RadWindow 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:
This being said, you should modify your code e.g in the following manner:
oWindowCust = $find(
'<%= rwCustomer.ClientID %>'
I hope that the provided explanation and resource are helpful .
the Telerik team