Related items widget has empty list when navigating the hierarchy

[fredvd]

Using Plone 4.3.14, testing p.a.widgets 1.91. Created a site, installed p.a.widgets in the control panel.

When I edit a page and try to select a related item, the top left most icon for navigating the content tree shows an empty drop down list and on the server a backtrace appears:

2017-05-16 15:00:54 ERROR Zope.SiteErrorLog 1494939654.580.994444735728 http://localhost:8080/plone/front-page/@@getVocabulary
Traceback (innermost last):
Module ZPublisher.Publish, line 138, in publish
Module ZPublisher.mapply, line 77, in mapply
Module ZPublisher.Publish, line 48, in call_object
Module plone.app.widgets.browser.vocabulary, line 75, in call
Module plone.app.widgets.browser.vocabulary, line 227, in get_vocabulary
Module plone.app.vocabularies.catalog, line 569, in call
Module plone.app.querystring.queryparser, line 28, in parseFormquery
Module plone.registry.registry, line 38, in getitem
KeyError: u'plone.app.querystring.operation.selection.any.operation'