fix  generic/mixins.py
@@ -80,6 +80,10 @@ class SectionMixin(object):
# when adding a new element, `element_id` can not be validated for uniqueness
postfix = 0
while True:
+ if not callable(getattr(form, "check_unique_element_id", None)):
+ form_=SectionForm
+ else:
+ form_=form
try:
form.check_unique_element_id(obj, element_id)
except ValidationError:
