"Value must not be null for Controls and
Behaviors"; a Lesson in Duh!

I spend about an hour trying to decrypt this message in the new AjaxToolKit for
3.5.

The simple, answer, and stupidity on my part, is that I was setting the
DynamicControlID for the DynamicPopulate to a PlaceHolder server control.  

This does not work due to the element naming once it is passed back to the
client. This is annoying because the PlaceHolder control is the natural choice
for recieving dynamic content.

So if you are plagued with this error, make sure that the control you are using
to recieve the DynamicContent is not a PlaceHolder control.