Fix PHPStan L2 error "Property Foo::$bar has unknown class Baz as its type."

Problem/Motivation

Amongst the "new" errors found when running PHPStan on level 2 is: Property Foo::$bar has unknown class Baz as its type.

This child-issue exists to fix all of those.

Steps to reproduce

- Run PHPStan on level 2 and see the above error amongst all others.

Proposed resolution

- Solve all of the the above mentioned reported errors.
- Run PHPStan on level 2 and don't see the above mentioned error any more.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet