Cannot add cells to a new ontology

Attempting to add individual cell terms to an active ontology file fails with the following error:

to replicate:
1. create an ontology file
2. set as active ontology
3. navigate to a PRECISION cell term.
4. select "Add term to active ontology"
5. see error.

Start by reproducing the five steps in the issue: create and activate an ontology file, navigate to a PRECISION cell term, and select “Add term to active ontology.” Use the displayed error to locate the failing entry point. Done means individual cell terms can be added to the active ontology without the error.

Written by the indexing model from the issue text.