Cannot add entries to empty vocabulary

1. install nuxeo
2. install webui addon
3. go to Administration / Vocabularies
4. select languages vocabulary
5. delete all the entries
6. add a new entry
7. observe it fails, the entry is not created and an error is in the Javascript console (see stacktrace.png)

Expected result: the entry should be added and no error should be raised.

This seems to be related to the _createfunction expecting a non-empty vocabulary:
https://github.com/nuxeo/nuxeo-web-ui/blob/9.10/elements/directory/nuxeo-vocabulary-management.html#L361