InvalidArgumentException on saving default values

Might this be resolved with 📌 Remove the special handling of 'class' attribute in favour of classed plugins Needs work ?
I think we should have a test-only test to check this as first step? That would make this more clear.

Needs tests to reproduce this please and ensure it's fixed, once 📌 Remove the special handling of 'class' attribute in favour of classed plugins Needs work is resolved.

Same error when trying to set default value in link field inside Paragraph