Controls that save arrays cause PHP Notices

[aristath]

Example:

PHP Notice:  Array to string conversion in /srv/www/wordpress-default/wp-includes/formatting.php on line 1025
PHP Stack trace:
PHP   1. {main}() /srv/www/wordpress-default/wp-admin/customize.php:0
PHP   2. do_action($tag = 'customize_controls_print_footer_scripts', $arg = *uninitialized*) /srv/www/wordpress-default/wp-admin/customize.php:208
PHP   3. WP_Hook->do_action($args = array (0 => '')) /srv/www/wordpress-default/wp-includes/plugin.php:453
PHP   4. WP_Hook->apply_filters($value = NULL, $args = array (0 => '')) /srv/www/wordpress-default/wp-includes/class-wp-hook.php:323
PHP   5. WP_Customize_Manager->customize_pane_settings('') /srv/www/wordpress-default/wp-includes/class-wp-hook.php:298
PHP   6. WP_Customize_Control->json() /srv/www/wordpress-default/wp-includes/class-wp-customize-manager.php:3590
PHP   7. Kirki_Control_Sortable->to_json() /srv/www/wordpress-default/wp-includes/class-wp-customize-control.php:322
PHP   8. WP_Customize_Control->to_json() /srv/www/wordpress-default/wp-content/plugins/kirki/controls/sortable/class-kirki-control-sortable.php:72
PHP   9. WP_Customize_Control->get_content() /srv/www/wordpress-default/wp-includes/class-wp-customize-control.php:304
PHP  10. WP_Customize_Control->maybe_render() /srv/www/wordpress-default/wp-includes/class-wp-customize-control.php:366
PHP  11. WP_Customize_Control->render() /srv/www/wordpress-default/wp-includes/class-wp-customize-control.php:401
PHP  12. WP_Customize_Control->render_content() /srv/www/wordpress-default/wp-includes/class-wp-customize-control.php:414
PHP  13. esc_attr($text = array (0 => 'option6', 1 => 'option1')) /srv/www/wordpress-default/wp-includes/class-wp-customize-control.php:611
PHP  14. wp_check_invalid_utf8($string = array (0 => 'option6', 1 => 'option1'), $strip = *uninitialized*) /srv/www/wordpress-default/wp-includes/formatting.php:3949