Add order items modal is empty for a new order type

Describe your bug or feature request.

If a bug, provide steps to reproduce it from a clean install.

1. Create a new order type from the UI
2. Create a new order of that type
3. Click on "Add order items" below the order items table
4. Notice the modal is empty

Also, I realized the order_items field still uses the Inline Entity Form widget by default:

      ->setDisplayOptions('form', [
        'type' => 'inline_entity_form_complex',
        'weight' => 0,
        'settings' => [
          'override_labels' => TRUE,
          'label_singular' => t('order item'),
          'label_plural' => t('order items'),
          'removed_reference' => 'delete',
        ],
      ])

I thought we changed that, and made our new order items widget the default.