Drupal\Core\Render\Component\Exception\InvalidComponentException: [slots.messages] Array value found, but an object is required

Problem/Motivation

Drupal\Core\Render\Component\Exception\InvalidComponentException: [slots.messages] Array value found, but an object is required in Drupal\Core\Theme\Component\ComponentValidator->validateDefinition() (line 122 of core/lib/Drupal/Core/Theme/Component/ComponentValidator.php).