Unable to display related posts: Fatal error

I have two post types: 'Conversation' and 'Person'.

When creating a Conversation, a user can select people to be related to the Conversation.
I created the relationship like this:

add_action( 'mb_relationships_init', function() {
    MB_Relationships_API::register( array(
        'id'   => 'connect_conversations_to_people',
        'from' => 'conversation',
        'to'   => 'person',
    ) );
} );

This works: the user can indeed select People when creating a Conversation post.

In the Page Template for displaying the Conversation, I want to display the People who are related to the Conversation. I've basically copied code from the documentation for MB Relationships:

$connected = new WP_Query( array(
            'relationship' => array(
                'id'   => 'posts_to_pages',
                'from' => get_the_ID(), // You can pass object ID or full object
            ),
            'nopaging'     => true,
        ) );
        while ( $connected->have_posts() ) : $connected->the_post();
        ?>
            <a>"><?php the_title(); ?></a>
        <?php
        endwhile;
        wp_reset_postdata();

This yields the fatal error:

Fatal error: Uncaught Error: Call to a member function get_db_field() on null in /var/www/mysite.localdev/wp-content/plugins/meta-box-aio/vendor/meta-box/mb-relationships/inc/query/normalizer.php on line 75

What could be happening here?