Latest plugin update throws fatal error

Hello Fabian,

Thank you for reaching out.

This is a very weird issue. It looks like a field group (meta box) on the site is registered with null id. For example:

add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
    $meta_boxes[] = [
        'title'      => 'Event details',
        'post_types' => 'post',
        'id'         => null, // this causes the error
        'fields'     => [
            ...
        ],
    ];

    return $meta_boxes;
} );

Can you please recheck all the field groups registered on the site if one has null ID?