Problem

1. Create task.
2. Add Coordinator group job.
3. Login as Coordinator
4. Create user job
5. try to change job step to step that differ from Coordinator group job

In master that will result in 500 error on front

Solution

Use existing message for this case: 

Coordinator group user cannot be assigned to a job before the Coordinator group job has been created

but change it for all places where it is used to:

You cannot assign a job for a user to this workflow step. Reason: You - as a job coordinator - have to be assigned to a job for this workflow step, in order to be able to forward the job to someone else. But you are not assigned to the currently selected workflow step.