Bug: backup field does not work inside a group field

Hi,

The code below triggers a fatal error in PHP 8 and a warning in PHP 7.4 or lower.

add_filter( 'rwmb_meta_boxes', function ( $meta_boxes ) {
    $meta_boxes[] = [
        'id'             => 'general',
        'title'          => 'General',
        'settings_pages' => 'my-options',
        'context'        => 'normal',
        'fields'         => [
            [
                'id'     => 'my_group',
                'type'   => 'group',
                'clone'  => false,
                'fields' => [
                    [
                        'name' => 'Backup',
                        'type' => 'backup',
                    ],
                ]
            ]
        ]
    ];
    return $meta_boxes;
} );

( ! ) Fatal error: Uncaught TypeError: get_class(): Argument #1 ($object) must be of type object, null given in /(...)/meta-box-aio/vendor/meta-box/mb-settings-page/src/BackupField.php on line 4

It works fine without the group field.

Here's a quick video displaying the bug: https://youtu.be/GylF4X20kyE

And a test plugin: https://dl.dropbox.com/s/6weozndvcc6jbo3/test-plugin.zip

I look forward to the resolution of the issue, thanks.