Custom Admin Column: Uncaught Error: Class "MBAC\Post" not found

I am using the documentation here to create custom code to include the slug of the post in the admin columns.
https://docs.metabox.io/extensions/mb-admin-columns/#custom-admin-columns

The first error I get is:
PHP Fatal error: Uncaught Error: Class "MBAC\Post" not found

What am I missing?

I am running Metabox 5.10.3 and AIO 1.30.5

<div>
class Prefix_Custom_Admin_Columns extends \MBAC\Post {
    public function columns( $columns ) {
        $columns  = parent::columns( $columns );
        $position = 'after';
        $target   = 'title';
        $this->add( $columns, 'post_name', 'Slug', $position, $target );
        // Add more if you want
        return $columns;
    }
    public function show( $column, $post_id ) {
        switch ( $column ) {
            case 'post_name':
                echo $post->post_name;;
                break;
            // More columns
        }
    }
}
</div>

<div>
Here is the function:
add_action( 'admin_init', 'prefix_add_custom_columns', 20 );
function prefix_add_custom_columns() {
    require_once 'custom.php';
    $post_type = 'post';
    new Prefix_Custom_Admin_Columns( $post_type, [] );
}
</div>