To get a null object you have probably attempted to get a reference to the object with the $find() method before the controls are initialized.
On the difference between these methods you can read in the MSDN articles on $find(), getElementById() and in this blog post.
the Telerik team
Registration for Q1 2011 What’s New Webinar Week is now open. Mark your calendar for the week starting March 21st and book your seat
for a walk through all the exciting stuff we ship with the new release!