Don't assume plugin config.yml has a parent key as it's not required (and for example lcnaf does not).

To replicate

Do something like:

git clone --recurse-submodules https://github.com/archivesspace/archivesspace aspace-with-submodules
cd aspace-with-submodules
./build/run build-dist

How Has This Been Tested?

As above and locally updated the hello_world example plugin to test the conditions (no parents and with parents.agent).

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have authority to submit this code.
• I have added tests to cover my changes.
• All new and existing tests passed.