500 Internal Server Error on `POST /api/v1/optimization-units HTTP/1.1`

I just tried to create an optimization unit, but after I click the `Add` button, nothing happened.

Then I looked into docker logs, and here they are:
```
2025-12-23 18:36:56 | INFO     | Adding optimization unit Test (None), Active: False
INFO:     172.19.0.1:0 - "POST /api/v1/optimization-units HTTP/1.1" 500 Internal Server Error
```
