WSOD on config page if field_ui is not enabled

Problem/Motivation

While testing 📌 Fix the issues reported by phpcs Needs review with the Minimal installation profile, I noticed that the configuration page /admin/structure/paragraphs_type/ptoc generated a WSOD. This is because the Minimal profile does not enable the field_ui module.

Steps to reproduce

1. Install Drupal with the minimal installation profile.
2. Enable the ptoc module and its dependencies.
3. Visit /admin/structure/paragraphs_type/ptoc.

The result is a WSOD, with this error in the logs:

Symfony\Component\Routing\Exception\RouteNotFoundException: Route "entity.entity_view_display.paragraph.view_mode" does not exist. in Drupal\Core\Routing\RouteProvider->getRouteByName() (line 206 of /var/www/html/core/lib/Drupal/Core/Routing/RouteProvider.php).

Proposed resolution

Some sites do not enable the field_ui module on production, so adding that as a dependency is not a good option.

The code in PtocConfigForm::buildForm() should check that the route exists before generating the URL:

          $url = Url::fromRoute('entity.entity_view_display.paragraph.view_mode', [
            'paragraphs_type' => $bundle,
            'view_mode_name' => 'ptoc',
          ]);

Remaining tasks

User interface changes

No WSOD when field_ui is not enabled.

API changes

None

Data model changes

None