RelationList with SelectFieldWidget: ValueError: Circular reference detected

Plone Support Add-on Development
3

I have been using this monkey patch for a long time:

  <monkey:patch description="Fix circular reference when field is RelationValue"
        class="plone.app.content.utils"
        original="custom_json_handler"
        replacement=".monkey_patches.pac_utils_custom_json_handler"
        />

def pac_utils_custom_json_handler(obj):
    #log.info('%s custom_json_handler called' % get_linenumber())
    if obj == Missing.Value:
        return None
    if type(obj) in (datetime.datetime, datetime.date):
        return obj.isoformat()
    if type(obj) == DateTime:
        dt = DateTime(obj)
        return dt.ISO()
    if type(obj) == set:
        return list(obj)
    # 20170418 fixed Circular reference
    # return the id of the related object(s)
    if type(obj) == RelationValue:
        return obj.to_path.split('/')[-1]
    if type(obj) == RelationList:
        return list(item.to_path.split('/')[-1] for item in obj)
    return obj
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