Create/Edit Project: unclear mandatory input

[Clairebaer]

Describe the bug
If a project is saved with missing information, it is not shown in the overview. In one case the overview cannot be opened but throws DivisionByZero-Error.

Case 1:
To Reproduce
Steps to reproduce the behavior:

1. Open an existing (closed) project.
2. Clear end of project. (It might be possible that no 'Umfang'was filed, but that wasn't changed during this specific case.)
3. Save. ('Saved' note shows up.)
4. Click on 'Projects'.
5. Project is not shown, number of closed projects decreased for one.
6. Filter for 'All'.
7. Search manually for a part of the project name (e. g. 'test)
8. Instead of showing no result, a DivisionByZero-error occurs.

Expected behavior
The new project is shown on top of the list with active/all projects.

Screenshots

Case 2:

To Reproduce
Steps to reproduce the behavior:

1. Create a project.
2. Fill out mandatory information, including 'Umfang'.
3. Save. ('Saved' note shows up.)
4. Click on 'Projects'.
5. Project is shown, number of projects did increase.
6. Re-open project.
7. Clear 'Umfang' and save.
8. Click on 'Projects'.
9. Project is not shown, number of projects did decrease.
10. Search manually for a part of the project name.
11. Instead of showing no result, a DivisionByZero-error occurs
12. When accessing the projects-page later (without any further changes), a DivisionByZero error is shown immediately without showing the list of projects.

Expected behavior
The new project is shown on top of the list with active/all projects.

Screenshots

System (please complete the following information):

• version: 2.1.9

Additional context
Add any other context about the problem here.