Expanded class hierarchy of NullGenerator

class \Symfony\Component\Routing\Generator\UrlGenerator implements ConfigurableRequirementsInterface, UrlGeneratorInterface
- class \Symfony\Cmf\Component\Routing\ProviderBasedGenerator implements VersatileGeneratorInterface
  - class \Drupal\Core\Routing\UrlGenerator implements PathBasedGeneratorInterface
    class \Drupal\Core\Routing\NullGenerator
  - class \Symfony\Cmf\Component\Routing\ContentAwareGenerator
  - class \Symfony\Cmf\Component\Routing\Tests\Routing\TestableProviderBasedGenerator