Choice field not required: Constraint not satisfied

I created a behavior with the following field definition:

mfolder = schema.Choice(
    title=u"Source Folder Mobile",

                  , RelatedItemsFieldWidget
                  , pattern_options={
        'recentlyUsed': True,
        'mode': 'search',
        'maximumSelectionSize': 1,
        'selectableTypes': ['Folder'],

The choice should not be required. It works fine if I select a content relation, but leaving it empty responses with:
"Constraint not satisfied"


What am I doing wrong.

Plone 5.2.9 3.2.4

Try using the RelationChoice field from z3c.relationfield.schema

This may be helpful: 37. Relations – Mastering Plone 5 Development — Plone Training 2022 documentation

Great, that works. Thanks for the help!!!