Render #pre_render callbacks must be methods of a class that implements TrustedCallbackInterface

Problem/Motivation

Using Drupal 9 when creating a context with block reaction, I have the error below:

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\context\Plugin\ContextReaction\Blocks::preRenderBlock. 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

The Blocks must implement the TrustedCallbackInterface.

See record changes: https://www.drupal.org/node/2966725