You should use the ClientID of the control:
What happens in your case is that this code returns tha ID of the first
DOM element whose ID ends with RadWindow1.
Before opening the RadWindow for the first time, this is the <div> used for generating the IScriptControl instance.
Since the correct ID is no longer passed to $find(), you get a null reference.
If server code blocks cannot be used in your app like this, I suggest adding a replace(
) call to remove
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