ComboBox Image Skin

[OrlandoII]

Hi, I tryed in several ways to make an image skin for the combobox component but I always have the following error (in debug console):

TypeError: Error #1009: Cannot access a property or method of a null object reference.
at com.bit101.components::List/draw()[C:\as3\com\bit101\components\List.as:162]
at com.bit101.components::Component/onInvalidate()[C:\as3\com\bit101\components\Component.as:220]

I've investgated and it seems that the _scrollbar component is null, so I tried to make skins also for the "panel" and the "scrollBar" components but without success (it seems that the function skinPartAdded is not called from the parent object).

So, do you have any hint or working example?

Thanks and regards,
Orlando