Creating a List with just text items works fine:
mainListboxEntries = [u'One', u'Two]
mainListbox = appuifw.Listbox(mainListboxEntries,self.handleMainListboxEvent)
Upon exiting the application I set both 'mainListboxEntries' and 'mainListbox' to 'None' so I don't get the 'Program closed: Python' error when exiting Python.

If I create a list with Icons I *DO* get the 'Program closed: Python' error, even though I'm sure I release all native resources (I also set all icons to 'None' upon exit).
checkIcon = appuifw.Icon(u"c:\\icons.mbm", 0, 0)
mainListboxEntries = [(u'Check', checkIcon)]
If I create the icons but not use them for the List I don't get the 'Program closed: Python' error. I get the feeling that a List does not release its resources properly.