Page Validation and ModalPopupExtender

I, like others before me, am having difficulty performing validation of page controls when also using a ModalPopupExtender on the same Page. I have tried the solutions offered in various Asp.Net form posts as well as those found in StackOverflow. None work as intended. In fact, several cause HttpExceptions.
A simple example is a DropDownList with a RequiredFieldValidator associated with it. Also on the Page is a LinkButton (the "submit" button), a Panel, and a ModalPopupExtender. I need to ensure that the DDL validation is performed, and, if it succeeds, I want the ModalPopupExtender to display the Panel. 
Any help would be greatly appreciated.