Null Error in PopUpManagerImpl.as when removing a display object

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()

    at mx.managers::PopUpManagerImpl/removeModalPopUpAccessibility()

    at mx.managers::PopUpManagerImpl/popupRemovedHandler()

    at flash.display::DisplayObjectContainer/removeChild()

    at mx.managers::SystemManager/http://www.adobe.com/2006/flex/mx/internal::rawChildren_removeChild()

    at mx.managers::SystemManager/removeChild()

    at mx.managers::PopUpManagerImpl/removePopUp()

    at mx.managers::PopUpManager$/removePopUp()

..................

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