[flexcoders] PopUpManager Problem

[Mark]

I need some help with an error I am getting when using the popupmanager.
Essentially the problem I am having is if I dynamically load SWFs from
my main application and more than one of the SWF uses the popupmanager I
receive an error. The specific error I am getting is:
TypeError: Error #1009: Cannot access a property or method of a null
object reference.
     at mx.managers::PopUpManager$/addPopUp()
     at mx.controls::Alert$/show()
     at Tab2/::onClick()
     at Tab2/___Button1_click()

My test app to reproduce this problem is essentially on the main
application a tabnavigator with two VBox as the tabs. The SWF files then
get loaded dynamically into those VBox. The two SWF files only contain a
button to press and it displays an Alert (the builtin Alert uses
PopUpManager).
Has anyone seen this error before and can shed some insight?

Thanks in advance.
--Mark

Below is my code to reproduce this:

PopUpApp.as
package
{
     import mx.core.Application;
     import flash.events.Event;

     public class PopUpApp extends Application
     {
         [Bindable] public var debug:Boolean;

         public function PopUpApp()
         {
             super();    //Invoke the Application superclass
             this.addEventListener("creationComplete",
onCreationComplete);
         }

         private function onCreationComplete(event:Event):void
         {
             debug = (this.url.indexOf("-debug.swf")) > -1 ? true :
false;
         }
     }
}

PopUpProblem.mxml
<?xml version="1.0" encoding="utf-8"?>
<local:PopUpApp
     xmlns:local="*"
     xmlns:mx="http://www.adobe.com/2006/mxml";
     layout="absolute"
     width="100%"
     height="100%"
     creationComplete="onCreationComplete();">

     <mx:Script>
         <![CDATA[
             import mx.events.ModuleEvent;
             import mx.modules.ModuleManager;
             import mx.modules.IModuleInfo;

             public var tab1Mod:IModuleInfo;
             public var tab2Mod:IModuleInfo;

             private function onCreationComplete():void
             {
                 if (this.debug)
                 {
                     tab1Mod = ModuleManager.getModule("Tab1-debug.swf");
                     tab2Mod = ModuleManager.getModule("Tab2-debug.swf");
                 }
                 else
                 {
                     tab1Mod = ModuleManager.getModule("Tab1.swf");
                     tab2Mod = ModuleManager.getModule("Tab2.swf");
                 }

                 tab1Mod.addEventListener(ModuleEvent.READY,
tab1ModReady);
                 tab2Mod.addEventListener(ModuleEvent.READY,
tab2ModReady);
                 tab1Mod.load();
                 tab2Mod.load();
             }

             private function tab1ModReady(event:ModuleEvent):void
             {
                 tab1.addChild(tab1Mod.factory.create() as
DisplayObject);
             }

             private function tab2ModReady(event:ModuleEvent):void
             {
                 tab2.addChild(tab2Mod.factory.create() as
DisplayObject);
             }
         ]]>
     </mx:Script>

     <mx:TabNavigator width="100%" height="100%">
         <mx:VBox id="tab1" label="Tab 1" width="100%" height="100%" />
         <mx:VBox id="tab2" label="Tab 2" width="100%" height="100%" />
     </mx:TabNavigator>

</local:PopUpApp>


Tab1.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Module
     xmlns:mx="http://www.adobe.com/2006/mxml";
     layout="horizontal"
     percentWidth="100"
     percentHeight="100">

     <mx:Script>
         <![CDATA[
         import mx.controls.Alert;
         import flash.events.MouseEvent;

         private function onClick(event:MouseEvent):void
         {
             mx.controls.Alert.show("Tab1 Alert");
         }
         ]]>
     </mx:Script>

     <mx:Button label="Button" click="onClick(event);"/>

</mx:Module>

Tab2.mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Module
     xmlns:mx="http://www.adobe.com/2006/mxml";
     layout="horizontal"
     percentWidth="100"
     percentHeight="100">

     <mx:Script>
         <![CDATA[
         import mx.controls.Alert;
         import flash.events.MouseEvent;

         private function onClick(event:MouseEvent):void
         {
             mx.controls.Alert.show("Tab2 Alert");
         }
         ]]>
     </mx:Script>

     <mx:Button label="Button" click="onClick(event);"/>

</mx:Module>