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

Problem/Motivation

When adding the "Show rss news" block to the dashboard, an error occurs:

Drupal\Core\Security\UntrustedCallbackException: Render #lazy_builder callbacks must be methods of a class that implements \Drupal\Core\Security\TrustedCallbackInterface or be an anonymous function. The callback was Drupal\dashboards\Plugin\Dashboard\RssNews::lazyBuildPreRender. See https://www.drupal.org/node/2966725 in Drupal\Core\Render\Renderer->doTrustedCallback() (line 96 of core/lib/Drupal/Core/Security/DoTrustedCallbackTrait.php).

Proposed resolution

Implement the TrustedCallbackInterface, as explained in https://www.drupal.org/node/2966725.