MM_wp

1. Without the meta_box_cb it all works as expected
2. I added that setting because it was better for me to have the list of checkbox to select rather than the “type in” metabox. It works without problems in the edit page, but breaks in the quick edit where the “type in” is showed and obv fails the validation.
3. To prevent: IMO a notice that says “non-hierarchical taxonomies can’t be used in Quick Edit with post_categories_meta_box callback” would be nice.