Null Error in PopUpManagerImpl.as when removing a display object

Question

Hi all,
We are in the middle of migration a flex 3.5 application to flex 4.6 with flash builder 4.5.
I have done basics and managed the get the application loading. However, when I try to remove a popup I get the following error.
TypeError: Error #1009: Cannot access a property or method of a null object reference.
    at mx.managers::PopUpManagerImpl/handleAccessibilityForNestedPopups()[E:\dev\4.y\frameworks\ projects\framework\src\mx\managers\PopUpManagerImpl.as:1533]
    at mx.managers::PopUpManagerImpl/removeModalPopUpAccessibility()[E:\dev\4.y\frameworks\proje cts\framework\src\mx\managers\PopUpManagerImpl.as:1474]
    at mx.managers::PopUpManagerImpl/popupRemovedHandler()[E:\dev\4.y\frameworks\projects\framew ork\src\mx\managers\PopUpManagerImpl.as:1222]
    at flash.display::DisplayObjectContainer/removeChild()
    at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::rawChildren_removeChild()[E:\dev\4.y\frameworks\projects\framework\src\mx\managers\SystemManager.as:2144]
    at mx.managers::SystemManager/removeChild()[E:\dev\4.y\frameworks\projects\framework\src\mx\ managers\SystemManager.as:1787]
    at mx.managers::PopUpManagerImpl/removePopUp()[E:\dev\4.y\frameworks\projects\framework\src\ mx\managers\PopUpManagerImpl.as:633]
    at mx.managers::PopUpManager$/removePopUp()[E:\dev\4.y\frameworks\projects\framework\src\mx\ managers\PopUpManager.as:230]
underneathPopUpData.owner.accessibilityProperties.silent = false;
in PopUpManagerImpl.as:1533 and PopUpManagerImpl.as:1519
where accessibilityProperties is null
I have looked over the web to any similar threads to no avail.
Am I missing something?
thank you very much

Answer 1

YES!!!!!   FINALLY!!!!!
I've been looking for WEEKS for this solution!!!   It was driving me CRAZY!!
Why would my program run fine on some computers and terribly on others...  The newer machines always had the problems, and the older ones never had any problems.  THANK YOU SO MUCH FOR FIGURING THIS OUT!!  I'm SO happy!!!  Yay!!!!!!
Simply checking "Generate Accessible SWF" on the Flex Compiler page of the Options screen fixed everything.  Whew!!!
*weight of the world lifted off my shoulders*