Steps to reproduce:
1. Start opencast locally, or with a fast backend so HTTP answers arrive fast.
2. Create a new event, publish it somewhere (doesn't matter where).
3. Click on "delete" in the events table for the corresponding event.
4. The "retract" modal pops up. Its "Submit" button is disabled.
The submit button is disabled, even after the list of workflows and publication channels has been successfully retrieved.
The submit button might be disabled as long as the "get workflows" request is in flight, but afterwards, it's supposed to be enabled.
I noticed that this doesn't happen with "slower" systems or if I manually throttle requests (for example, using the appropriate developer tool in chrome). I suspect this is a timing bug somewhere in `retractEventController.js` and/or `processing.js`.