Cannot add cells to a new ontology

[stappan]

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.

[stappan]

@tgbugs is this front end or back end?