It seems like the wrong additional fields mixin was used for Koha::Acquisition::Booksellers in bug 38262, Koha::Object::Mixin::AdditionalFields when it should have been Koha::Objects::Mixin::AdditionalFields. Here is a fix for that.
Created attachment 192976 [details] [review] Bug 41816: Wrong additional fields base class for Booksellers Fix additional fields base class for Koha::Acquisition::Booksellers Test plan: 1) Perform a vendor search, it should result in a crash. 2) Apply patch. 3) Perform the search again and verify the problem has been resolved.