@mtrebron thank you, I saw the discussion here. In the final post you said you worked around the issue by monkey patching toWidgetValue in plone.app.z3cform.converters.AjaxSelectWidgetConverter.
As I said in the initial post, the only way I found to get this working is by monkey patching the same method by adding the following code at the beginning:
if value == (None,):
value = None
How did you patch it?
I have the same code working perfectly in production on a Plone 4.2 installation for five years now. Here you can read another example, and here another one.
There's also an example on the official documentation of a Source Binder with SimpleVocabulary.
Me too.
@1letter this is basically the same code I wrote a few posts before, but as I already said I cannot use vocabularies because I need to fetch data from an external database and the number of records is very high.
Davide