You are working in an ASP.NET AJAX environment and in AJAX, all ASP.NET AJAX controls, including RadControls are created after the page is loaded - you can check that by examining the HTML dump.
What happens is that when you try to execute the script on page's load, RadRotator
will still not be rendered on the page which will cause the error.
To avoid the problem, you should call the script on a later stage - for example in ASP.NET AJAX's generic pageLoad() function - it is called when all the controls are loaded
//your code here
I hope this helps.
the Telerik team
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