Field using StaticCatalogVocabulary with AjaxSelectFieldWidget raises AttributeError on nonexisting value

[pbauer]

When a content item that was stored in a schema.List field with AjaxSelectFieldWidget and a StaticCatalogVocabulary as vocabulary a missing content item raises:

AttributeError: http://localhost:8080/paediatrics/Plone/team/@@edit
Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 167, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 376, in publish_module
  Module ZPublisher.WSGIPublisher, line 271, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module Products.PDBDebugMode.wsgi_runcall, line 60, in pdb_runcall
  Module plone.z3cform.layout, line 63, in __call__
  Module plone.z3cform.layout, line 47, in update
  Module plone.dexterity.browser.edit, line 55, in update
  Module plone.z3cform.fieldsets.extensible, line 65, in update
  Module plone.z3cform.patch, line 30, in GroupForm_update
  Module z3c.form.group, line 141, in update
  Module z3c.form.group, line 52, in update
  Module z3c.form.group, line 48, in updateWidgets
  Module z3c.form.field, line 274, in update
  Module plone.app.z3cform.widget, line 441, in update
  Module z3c.form.browser.text, line 36, in update
  Module z3c.form.browser.widget, line 171, in update
  Module Products.CMFPlone.patches.z3c_form, line 46, in _wrapped
  Module z3c.form.widget, line 132, in update
  Module plone.app.z3cform.converters, line 203, in toWidgetValue
AttributeError: 'NoneType' object has no attribute 'token'

Field example (basically the same as https://github.com/collective/example.contenttype/blob/master/src/example/contenttype/example.py#L660):

myfield = schema.List(
    title="My field"
    value_type=schema.Choice(
        vocabulary=StaticCatalogVocabulary({'portal_type': 'mytype'})),
    required=False,
    default=[],
)
directives.widget(
    'myfield',
    AjaxSelectFieldWidget,
    vocabulary=StaticCatalogVocabulary({'portal_type': 'mytype'})
)

The contract of ITerm says thai getTerm (called intoWidgetValue) it should instead raise a LookupError which can bei either ignored by the DataCoverter or shown.

[pbauer]

I'm not 100% sure if this is the right solution but with the change in #71 only the uuid is displayed when editing:

When saving again a validation-error is shown ("Wrong type for container") which seems correct I think.
But a general question raises it's head here: How do we handle values from vocabularies stored in fields that are no longer valid because these values (be it content or other options) are not longer available?