The dataProvider is providing an ArrayCollection of String
values where the first value is a blank string "". There is a
StringValidator associated with the ComboBox to make sure the user
makes a selection.
The error occurs when you tab to the ComboBox, and then use
the down arrow key on the keyboard. This error does not occur when
you use the mouse to access the drop down list. In the debugger the
_downdown property is null.
TypeError: Error #1009: Cannot access a property or method of
a null object reference.
In the ComboBox.as the following line is raising an null
object reference error as the _dropdown property is null:
1848: if (oldIndex != _dropdown.selectedIndex)
1849: selectedIndex = _dropdown.selectedIndex;