AJAX HTTP error

This may be just due to my lack of familiarity with the module but I'm having an issue setting up a new field inheritance field. I'm using this module in conjunction with the recurring events → module.

Problem/Motivation

When clicking save on the Field Inheritance form I get the attached ajax error.

Steps to reproduce

1. Create new drupal site following the composer → instructions.
2. Create a new event series.
3. Add a text field to the event series.
4. Setup an event instance to inherit from the series. Screenshots attached.

Module Versions

drupal/field_inheritance - 2.0.0-rc5

drupal/recurring_events - dev-2.0.x