Adding field to fieldset introduced by other behavior not working, throwing an error

[zopyx]

Plone 5.2, plone.schemaeditor 2.0.20.

Screenshot shows the schemaeditor for xmldirector.connector with a custom behavior adding some fields through the "guideline" fielset. Trying to add a custom field to the field throws an error.

Traceback (innermost last):
  Module ZPublisher.WSGIPublisher, line 142, in transaction_pubevents
  Module ZPublisher.WSGIPublisher, line 295, in publish_module
  Module ZPublisher.WSGIPublisher, line 229, in publish
  Module ZPublisher.mapply, line 85, in mapply
  Module ZPublisher.WSGIPublisher, line 57, in call_object
  Module plone.z3cform.layout, line 63, in __call__
  Module plone.z3cform.layout, line 47, 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 145, in update
  Module plone.app.z3cform.csrf, line 22, in execute
  Module z3c.form.action, line 98, in execute
  Module z3c.form.button, line 315, in __call__
  Module z3c.form.button, line 170, in __call__
  Module z3c.form.form, line 257, in handleAdd
  Module z3c.form.form, line 265, in createAndAdd
  Module plone.schemaeditor.browser.schema.add_field, line 83, in add
  Module plone.schemaeditor.utils, line 52, in get_fieldset_from_index
IndexError: list index out of range

> /home/ajung/sandboxes/onkopedia_buildout-plone5/eggs/plone.schemaeditor-2.0.20-py3.7.egg/plone/schemaeditor/utils.py(50)get_fieldset_from_index()
-> index = int(index or 0) - 1
(Pdb) args
schema = <SchemaClass xmldirector.connector.connector.IConnector>
index = 6
(Pdb) n
> /home/ajung/sandboxes/onkopedia_buildout-plone5/eggs/plone.schemaeditor-2.0.20-py3.7.egg/plone/schemaeditor/utils.py(51)get_fieldset_from_index()
-> fieldsets = schema.queryTaggedValue(FIELDSETS_KEY, [])
(Pdb) 
> /home/ajung/sandboxes/onkopedia_buildout-plone5/eggs/plone.schemaeditor-2.0.20-py3.7.egg/plone/schemaeditor/utils.py(52)get_fieldset_from_index()
-> return fieldsets[index] if index >= 0 else None
(Pdb) pp fieldset
*** NameError: name 'fieldset' is not defined
(Pdb) pp fieldsets
[]

[frapell]

I have commited a fix for this in #92 and plone/mockup#1203 which prevents fields to be added into fieldsets from behaviors, since I don't think this was ever intended.