Error - Class 'Grav\Plugin\DOMDocument' not found

Error
Class 'Grav\Plugin\DOMDocument' not found

<?php
namespace Grav\Plugin;

use Composer\Autoload\ClassLoader;
use Grav\Common\Plugin;

/* SEE: https://learn.getgrav.org/16/plugins/plugin-tutorial */
use Grav\Common\Page\Collection;
use Grav\Common\Uri;
use Grav\Common\Taxonomy;
/* use Grav\Common\Event; */

use Grav\Common\Page\Page;
use RocketTheme\Toolbox\Event\Event;
use RocketTheme\Toolbox\File\File;
use Thunder\Shortcode\Shortcode\ShortcodeInterface;

use ReflectionProperty;

class MyCustomPlugin extends Plugin
{
    public function onFormProcessed(Event $event)
    {
      ...
      $dom = new DOMDocument();
      ...
    }
}

-- PHP Loaded Extensions
 With function get_loaded_extensions()

 bcmath        bz2           calendar      com_dotnet    Core          ctype        
 curl          date          **dom**           exif          fileinfo      filter       
 gd            gettext       gmp           hash          iconv         imap         
 intl          json          ldap          libxml        **mbstring**      mysqli       
 mysqlnd       openssl       pcre          PDO           pdo_mysql     pdo_sqlite   
 Phar          readline      Reflection    session       SimpleXML     soap         
 sockets       SPL           sqlite3       standard      tokenizer     xdebug       
 xml           xmlreader     xmlrpc        xmlwriter     xsl           Zend OPcache 
 zip           zlib         

$dom = new \DOMDocument();