fix handling of foreign custom fields

[Phill93]

Closes: #723

What's Changed

Handle foreign custom fields. The attribute _custom_field_data contains a dictionary so getattr fails with an AttributeError exception. My fix checks if related_object is a dictionary and handles that different.

To Do

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• Attached Screenshots, Payload Example
• Unit, Integration Tests
• Documentation Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design

[Kircheneer]

Hey @Phill93 and thanks for the PR!

Please

• add a change log fragment (see first failing CI)
• add a unit test that asserts this behaviour keeps working in the future

[jdrew82]

@Phill93 just wanted to see if you could address the requested changes from @Kircheneer?

[Phill93]

@jdrew82 Yes, I have that on the plan. I just don't have time for it right now.