A bug in Flex SDK 3.4?? FocusManager.focusInHandler()

I have recently upgraded from Flex SDK 3.3 to SDK 3.4. Now I suddenly get this error message throughout the application:

TypeError: Error #1009: Cannot access a property or method of a null object reference.     

at mx.managers::FocusManager/focusInHandler()[C:\autobuild\galaga\frameworks\projects\framework\src\mx\managers\FocusManager.as:601] 

It occurs fairly at random, but most often when I click on a tab in a tabNavigator. 
The problem is fixed when I add the attribute 'focussable="false"' to the tabNavigator, 
but I'd like to keep the tabNavigator focussable, and I definately can't make all the elements in my app unfocussable.
Someone told me that this is a bug in the SDK. Is that true? If so, is there a workaround other than 'focussable="false"'?

解决办法是 'focussable="false"