ActionScript 2.0 :: PopUpManager.createPopUp() - Content's Form Subclass Isn't Created?

I'm working on a Flash application that uses the PopUpManager to create a pop-up window. Example of the create code follows:

details_window = mx.managers.PopUpManager.createPopUp(_level0, mx.containers.Window, false, null, true);
details_window.closeButton = true;
details_window.title = "The Popup Window Title";[code].....

Here's where the problems appear: The pop-up's content, "details.swf", is itself a Flash Form Application. The main screen of this Form application has a custom AS2.0 class assigned to it called DetailsScreen. The stripped down (for example purposes) class is below:
import mx.controls.*;

class DetailsScreen extends mx.screens.Form
{
var details: DataGrid; // the extra space between the ':' and 'DataGrid' is to prevent a smiley () on the forum
var details_title:TextField;
var record_display_summary:TextField;[code].....

The problem is the trace output in the DetailsScreen.onLoad() method is never printed. The pop-up window appears, and the graphical content of the details.swf is presented. I've recursively traced into details_window.content to see if DetailsScreen.local_test_variable is present, but it's not. The only variables present are those that represent the graphical items on the stage.It's as though the custom Form class DetailsScreen is never being instantiated.