Drupal 8 - Menu preprocess hook to fetch linked nodes and access thumbnail field values

drupal_8_theme.theme

use Drupal\node\Entity\Node;

function ncea_preprocess_menu(&$vars, $hook) {
  // Subjects Nav
  if ($hook == 'menu__subjects') {
    foreach ($vars['items'] as $key => $item) {
      if ($vars['items'][$key]['below']) {
        foreach ($vars['items'][$key]['below'] as $learningAreaKey => $learningAreaItem) {
            $learningAreaUrl = $learningAreaItem['url'];

            // Add learning area thumb url
            if ($learningAreaUrl instanceof Drupal\Core\Url) {
              $nid = $learningAreaUrl->getRouteParameters()['node'];
              $learningAreaNode = Node::load($nid);
              $learningAreaNodeThumbUrl = file_create_url($learningAreaNode->field_thumbnail->entity->getFileUri());
              $vars['items'][$key]['below'][$learningAreaKey]['thumbnail'] = $learningAreaNodeThumbUrl;
            } 
        }
      }
    }
  }
}