EntityAdapter::getIterator requires undocumented methods

Problem/Motivation

The EntityAdapter class now assumes that all entities have a getIterator() method, which is not documented in the EntityInterface and not provided for Config Entities.

As as result, many Drupal pages (e.g. configure block) return a fatal error due to a missing method on a Config Entity that interacts with that page.

The website encountered an unexpected error. Please try again later.
Error: Call to undefined method Drupal\domain\Entity\Domain::getIterator() in Drupal\Core\Entity\Plugin\DataType\EntityAdapter->getIterator() (line 170 of core/lib/Drupal/Core/Entity/Plugin/DataType/EntityAdapter.php).

Drupal\Core\Entity\Plugin\DataType\EntityAdapter->getIterator() (Line: 146)
Drupal\Core\TypedData\Validation\RecursiveContextualValidator->validateNode(Object, Array, 1) (Line: 99)
Drupal\Core\TypedData\Validation\RecursiveContextualValidator->validate(Object, Array, NULL) (Line: 90)
Drupal\Core\TypedData\Validation\RecursiveValidator->validate(Object, Array) (Line: 285)
Drupal\Core\Plugin\Context\ContextDefinition->isSatisfiedBy(Object) (Line: 46)
Drupal\Core\Plugin\Context\ContextHandler->Drupal\Core\Plugin\Context\{closure}(Object)

Proposed resolution

Rewrite the method to understand that getIterator() is not required.

  /**
   * {@inheritdoc}
   */
  public function getIterator() {
    return isset($this->entity) ? $this->entity->getIterator() : new \ArrayIterator([]);
  }

If we don't make this change, then the EntityInterface and base classes must be updated.

Remaining tasks

Provide a patch.

User interface changes

None.

API changes

None.

Data model changes

None