Steps to reproduce:
1. You need an 4.0 Opencast system with migrated data from a 3.x version. If you don't have one, you can simulate the situation by emptying the mh_assets_properties table.
2. Make a call to any External API event endpoint that involves getting metadata (api/event/metadata or api/event?withmetadata=true).
The call returns an error 500
The call should succeed
Workaround (if any):
Run a workflow on the events which contains at least an "snapshot" operation. From then on, that event will not cause an exception.