What's the code on the line where the debugger stops in the focusInHandler?
Focusable renderers aren't officially supported by Spark List. I would make
sure focus moves to some other thing when the List goes away.
The error occurs in this code line:
var elt:IVisualElement = focusManager.getFocus() as IVisualElement;
As stated above it seems the error is related to focusing the browser window (for example clicking the taskbar and then clicking flash content in the browser)
Yes, there are all kinds of problems when you remove something from the screen that had focus (for instance, at that point you can't capture keyboard events either).
My code is sprinkled with so many things to make sure something is in focus, I tend to forget that I've done it.
In your code that changes the currentState, add
Another way to do is to do a focusManager.deactivate(); on all controls that can have focus before removing them from stage.