I found some weird implementation. The inheritance hierarchie is at follows: ToggleButtonBar -> ButtonBar -> NavBar
ToggleButtonBar overrides the accessors and mutators for _selectedIndex. But why is the Accessor implemented in ToggleButtonBar like: override public function get selectedIndex():int { return super.selectedIndex; //return _selectedIndex; } It seems like this can cause a runtime error since 2 _selectIndex fields exist: in ToggleButtonBar and NavBar and both are accessed.