Prepare exercise start wrong progressbar status

[tobias-lippert]

Describe the bug

To Reproduce

1. Create exam
2. Add students
3. Generate student exams
4. Prepare exercise start
5. See the progressbar
6. Preparing exercise start again, fixes the issue

Expected behavior

Progressbar correctly reflects progress

Screenshots

No response

Which version of Artemis are you seeing the problem on?

develop

What browsers are you seeing the problem on?

Chrome

Additional context

No response

Relevant log output