This problem is caused by the fact that the NativeWindow class does not create a system manager when it initialises – this is what is required to support all the Flex controls.

The best way around this is to sub-class the NativeWIndow so it creates a system manager before you add any controls to it.

There’s a code sample and explanation that lives here:

http://www.ben-morris.com/howto-add-flex-mx-controls-to-a-nativewindow-for-adobe-air