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

[yaffs-website] / web / core / modules / views / tests / modules / views_test_modal / views_test_modal.routing.yml

views_test_modal.modal:
  path: '/views-test-modal/modal'
  defaults:
    _controller: '\Drupal\views_test_modal\Controller\TestController::modal'
  requirements:
    _access: 'TRUE'