How to get current post ID in backend

I'm wondering how to get current editable post ID in block MB, because this doesn't work:

        [
            'name'          => 'Страны',
            'id'            => 'countries',
            'type'          => 'post',
            'post_type'     => 'page',
            'placeholder'   => 'Выберите страницу',
            'query_args'    => [
                'post_parent'           => @$_GET['post']
            ],
            'clone'         => true,
            'sort_clone'    => true,
            'add_button'    => '+',
        ],