Deleting a custom field that workflow post functions rely on breaks transitions

Issue Summary

If Workflow post functions are configured to use a custom field and that custom field is deleted, the transition that uses the post function will break. This in turn means tickets cannot be transitioned or created.

There are no warnings at this time to indicate that a custom field is used as part of a workflow transition. Once a custom field is deleted, there is no clean up process to remove configurations such as workflow post functions that rely on that field.

Steps to Reproduce

1. Create a custom field
2. Create a workflow post function that relies on the custom field, such as clearing the custom field on transition
3. Delete the custom field once the workflow is published
4. Try to create an issue

Expected Results

I am seeing three potential implementation:

1. Prevent deleting custom fields that workflow transitions rely on, whether as a validator or post function, alerting the user of the link
2. Warn the user that the custom field is linked to workflow transition post functions or validators and deleting it will cause issues
3. When a custom field is deleted, remove the workflow element that relied on the field as a self cleanup

Actual Results

Issues cannot be created via the Jira User Interface or in the case of Jira Service Management via the customer portal. Based on my testing the errors I found:

Jira UI: We're having some trouble creating the issue. Refresh the page and try again.

Customer portal: Your request could not be created. Please check the fields have been correctly filled in.

Workaround

When deleting custom fields that were previously used in workflow transitions such as post functions, ensure you update these to a new field or remove the function itself.