Plone 5.2 AttributeError: 'ReferenceBrowserPopup' object has no attribute '__of__'

[mauritsvanrees]

http://localhost:8080/Plone2/refbrowser_popup
Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 162, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 359, in publish_module
  Module ZPublisher.WSGIPublisher, line 262, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 63, in call_object
  Module archetypes.referencebrowserwidget.browser.view, line 212, in __call__
  Module archetypes.referencebrowserwidget.utils, line 217, in __call__
AttributeError: 'ReferenceBrowserPopup' object has no attribute '__of__'

This is in a customer project that I want to update from 4.3 to 5.2. I think I have customised a bit. But I get the above error when I try to use the popup on an edit form.

I'm not sure if we will keep Archetypes for this project, as stepping stone before we move to dexterity and Python 3. Might be double work: first get it to work with Archetypes, and then do the work to go to dexterity.