I'm helping a non-profit to keep a site working (made by different people over the years) and we recently upgraded WP from the latest 4.x version to 5.2.4 . A few days ago I found an issue with a custom post type that is registered in a home grown plugin.

The error is triggered when I navigate to the add new page for the custom post type talk or when I edit and click update in an existing talk. There are no messages logged on the WP debug log or email to the WP admin email address.

The code that registers the custom type is (simplified slightly)

function wdm_talk_post_type() {
        $labels = array(
                'name'                   => _x( 'Talks', 'post type general name', 'wdm_teacher' ),
                'singular_name'          => _x( 'Talk', 'post type singular name', 'wdm_teacher' ),
                'menu_name'              => _x( 'Talks', 'admin menu', 'wdm_teacher' ),
                'name_admin_bar'         => _x( 'Talk', 'add new on admin bar', 'wdm_teacher' ),
                'add_new'                => _x( 'Add New', 'teacher', 'wdm_teacher' ),
                'add_new_item'           => __( 'Add New Talk', 'wdm_teacher' ),
                'new_item'               => __( 'New Talk', 'wdm_teacher' ),
                'edit_item'              => __( 'Edit Talk', 'wdm_teacher' ),
                'view_item'              => __( 'View Talk', 'wdm_teacher' ),
                'all_items'              => __( 'All Talks', 'wdm_teacher' ),
                'search_items'           => __( 'Search Talks', 'wdm_teacher' ),
                'parent_item_colon'      => __( 'Parent Talks:', 'wdm_teacher' ),
                'not_found'              => __( 'No teachers found.', 'wdm_teacher' ),
                'not_found_in_trash' => __( 'No teachers found in Trash.', 'wdm_teacher' )
        );

        $args = array(
                'labels'                 => $labels,
                'description'            => __( 'Description.', 'wdm_teacher' ),
                'public'                 => true,
                'publicly_queryable'     => true,
                'show_ui'                => true,
                'show_in_menu'           => true,
                'query_var'              => true,
                'rewrite'                => array( 'slug' => 'talk' ),
                'capability_type'        => 'post',
                'has_archive'            => true,
                'hierarchical'           => false,
                'menu_position'          => null,
                'supports'               => array( 'title', 'editor', 'author', 'thumbnail', 'excerpt', 'comments' )
        );

        register_post_type( 'talk', $args );

}
add_action( 'add_meta_boxes', 'wdm_talk_meta_box' );

function wdm_talk_post_save($post_id,$post){
    // I removed the body of the function, to test this bug.
    // The original code stores some custom fields
}
add_action('save_post_talk','wdm_talk_post_save');

If I comment the line that adds the action save_post_talk, the site works fine, but the logic to store the custom fields is not triggered.

Can someone spot the issue? I'm not a wordpress guru (but do have plenty of experience programming in other languages).