Breadcrumb errors on menu_link_content entities due to nonexistent bundle entity

Problem/Motivation

preprocessBreadcrumb hook fails when loading a MenuLinkContent entity.

Other entities such as block_content might fail as well.

      $entity_type = $entity->getEntityType();
      $type_label = $entity_type->getSingularLabel();
      $bundle_key = $entity_type->getKey('bundle');

      if ($bundle_key) {
        $bundle_entity = $entity->get($bundle_key)->entity;
        $type_label = $bundle_entity->label();
      }

The problem is that these entities declare a bundle entity key but no bundle_entity_type. Such entity types get a plain string bundle field rather than an entity_reference, so $entity->get($bundle_key)->entity is always NULL.

Steps to reproduce

- Visit a page which has a menu_link_content entity on the breadcrumb route.
- Miserably fail.

Proposed resolution

Add a check for the existence of an actual entity.

Remaining tasks

Code it

User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet