The RelationField does not work with the RadioFieldWidget widget (v4.4.0). However, the RadioFieldWidget can be selected.

[Arkusm]

1. Create a new EasyForm
2. Enter a title an save
3. From the action menu, select "Edit Form Fields"
4. Click "Add field"
5. Create a new field of type RelationChoice and click "Add"
6. For the newly created field, go to "Settings"
7. Select the RadioFieldWidget via the advanced tab and save.
8. The form can no longer be displayed. It is also not possible to edit the fields again.

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 181, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 390, in publish_module
  Module ZPublisher.WSGIPublisher, line 284, in publish
  Module ZPublisher.mapply, line 98, in mapply
  Module ZPublisher.WSGIPublisher, line 68, in call_object
  Module plone.z3cform.layout, line 61, in __call__
  Module plone.z3cform.layout, line 45, in update
  Module plone.z3cform.fieldsets.extensible, line 62, in update
  Module plone.z3cform.patch, line 31, in GroupForm_update
  Module z3c.form.group, line 133, in update
  Module z3c.form.form, line 140, in updateWidgets
  Module z3c.form.field, line 278, in update
  Module z3c.form.browser.radio, line 84, in update
  Module z3c.form.browser.widget, line 162, in update
  Module z3c.form.widget, line 232, in update
  Module z3c.form.widget, line 224, in updateTerms
  Module zope.component._api, line 113, in getMultiAdapter
zope.interface.interfaces.ComponentLookupError: ((<EasyFormFieldsView at /.../for-bugreport/fields>, <WSGIRequest, URL=.../for-bugreport/@@fields/@@listing>, <collective.easyform.browser.fields.FieldsSchemaListing object at 0x7fde12fa9f90>, <z3c.relationfield.schema.RelationChoice object at 0x7fde0be87810 plone.supermodel.generated..testfield>, <RadioWidget 'form.widgets.testfield'>), <InterfaceClass z3c.form.interfaces.ITerms>, '')