list property and canadditem

I have a list<string> property that will either be a list or null.
When the list is null the + button is not visible which is what I want.

If the property is later changed to a list<string> the + button is still not visible

I do have my custon converter for this property and the ConvertTo() method I override does get called when the list property changes from null to a list.
But I would also like the + button to be visible when the list is not null and to not be visible when it is null.


Thanks in advance,
Mary