You have not provided us with your code, so we are not sure what you are doing and when you are running this code.
That said, the RadFormDecorator obeys the rules of MS AJAX controls - it is initialized as part of the MS AJAX page initialization.
My suggestion is to not run your code until the page is properly initialized. MS AJAX provides "hooks" for which to subscribe. The easiest option is:
//This method, if it exists is called automatically
//Some code here that will automatically be called by the MS AJAX framework upon page load
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