method provides you with a reference to any control. You must make sure that you properly typecast the returned value as FindControl
returns a reference to a Control class. For detailed information on the subject please make a search in MSDN.
On aside note, the provided information does not help us much in reproducing the error. I'm afraid we could not be of much help unless we reproduce the issue on our side. It will be best if you can open a support ticket and send us a simple running project (incl. your custom skin, CSS, images, DB backup if needed and so on) demonstrating the problem (and step-by-step instructions on doing so). In that way we can reproduce and pinpoint the problems you're facing on our side, understand the logic of your application and provide a solution.
the Telerik team