Updated Drupal to 8.6. This goes with the following updates because it's possible...

[yaffs-website] / vendor / chi-teck / drupal-code-generator / templates / d8 / service / twig-extension.services.twig

services:
  {{ machine_name }}.twig_extension:
    class: Drupal\{{ machine_name }}\{{ class }}
{% if di %}
    arguments: ['@example']
{% endif %}
    tags:
      - { name: twig.extension }