Render #pre_render callbacks must be methods of a class that implements \Drupal\Core\Security\TrustedCallbackInterface or be an anonymous function.

Drupal\Core\Security\UntrustedCallbackException: Render #pre_render callbacks must be methods of a class that implements \Drupal\Core\Security\TrustedCallbackInterface or be an anonymous function. The callback was Drupal\entity_counter_webform\Plugin\EntityCounterSource\SubmissionsSource::preRenderForm. See https://www.drupal.org/node/2966725 en Drupal\Core\Render\Renderer->doTrustedCallback() (linea 96 de /var/www/html/docroot/core/lib/Drupal/Core/Security/DoTrustedCallbackTrait.php)