I am making a php class to make and manage custom metaboxes. Somehow it won’t save the values from the field, it doesn’t even save a value if I hardcode the update_post_meta function eg update_post_meta(25, 'test_key', 'foo'). I’m guessing there is something wrong with my add_action but it fires, I think, because when I add a var_dump() or echo in the save function it says “Updating failed. The response is not a valid JSON response”