A uniform method for getting Entity Reference(s) from Entity fields of varying cardinalities.

Drupal8_GetEntityRef.php

<?php

/**
 * @param \Drupal\Core\Entity\FieldableEntityInterface $entity
 *  Entity that has fields.
 * @param String $field_name
 *  Field name for Entity Reference field.
 *
 * @return \Drupal\Core\Entity\EntityInterface[]
 *  Return an array of one or more referenced entities.
 *
 * @link https://drupal.stackexchange.com/a/186317/61513
 *  For singular entity references, Drupal's `EntityReferenceItem` class lacks
 *   a means of directly getting the Entity object that is referenced in the
 *   field.
 *  Note: Multivalued Reference fields use `EntityReferenceFieldItemList`, which
 *   implements a `referencedEntities()` function which returns an array of
 *   referenced entities just fine.

 * @since Drupal 8.0.x
 */
function _get_entity_ref(\Drupal\Core\Entity\FieldableEntityInterface $entity, String $field_name) {
  if ($entity->hasField($field_name)) {

    // Get entity field type & value if the entity has the field.
    $field_type = $entity->getFieldDefinition($field_name)->getType();
    $field = $entity->get($field_name);
    if ($field) {

      // If it is a singular entity ref, return it's ref'd Entity in an array.
      if ($field_type === 'entity_reference') {
        /** @var Drupal\Core\Field\EntityReferenceFieldItemList $field */

        // Get first/only reference item from field.
        /** @var \Drupal\Core\Field\Plugin\Field\FieldType\EntityReferenceItem $ref_item */
        $ref_item = $field->first();

        // Get entity reference from the reference item.
        /** @var \Drupal\Core\Entity\Plugin\DataType\EntityReference $entity_ref */
        $entity_ref = $ref_item->get('entity');

        // Get the Entity wrapper.
        /** @var \Drupal\Core\Entity\Plugin\DataType\EntityAdapter $entity_adaptor */
        $entity_adaptor = $entity_ref->getTarget();

        // Return the Entity value from the wrapper.
        return [$entity_adaptor->getValue()];
      }

      // If it is a multi-valued Entity ref, get all ref'd Entities in an array.
      elseif ($field_type === 'entity_reference_revisions') {
        /** @var Drupal\entity_reference_revisions\EntityReferenceRevisionsFieldItemList $field */
        return $field->referencedEntities();
      }
      
      // TODO: Throw error or logging here for "else" cases where it is not an Entity Reference field.
    }
  }

  // Returns an empty array to ensure a consistent return type.
  // TODO: Consider throwing or logging an exception if the field was missing.
  return [];
}