Fix EZP-22886: Adding WF event breaks positions if user is not valid

Link: https://jira.ez.no/browse/EZP-22886

## Description
If you remove the user of a validation Workflow (making the WF edit form not valid) and add a new WF event, this event order are messed up. This fix disables the possibility to add new events when you have existing invalid ones.

## Tests
Manual tests