Custom entity used in Field UI

Question

I would like to add a reference field to my custom entity to a stock entity (eg. user) using the Field UI and the stock entity reference field (not my own custom field). The custom entity is perfectly functional. However, when I try to add it to another entity, I get an error on the Field UI page, from Drupal\Component\Utility\Html::escape() which leads back to Drupal\Core\StringTranslation\TranslatableMarkup.php:195. Basically, what happens is that the placeholder function (Drupal/Component/Render/FormattableMarkup.php:187) receives a

['@label' => NULL]

item while it tries to assemble the current entities for the display of the defaults. I do have a label entry in my entity_keys that points to the field I use as a name. I also have common_reference_target set to TRUE.

Is there something else I miss to be able to use my entity with the stock reference field?

Not that I would condone such behavior but if I happen to temporarily change FormattableMarkup.php:187 to

$args[$key] = static::placeholderEscape($value ?? '');

the reference field can be added and it will keep working, even if this modification in core is reverted later.

Update:

 * @ContentEntityType(
 *   id = "country",
 *   label = @Translation("Country"),
 *   label_collection = @Translation("Countries"),
 *   label_singular = @Translation("country"),
 *   label_plural = @Translation("countries"),
 *   label_count = @PluralTranslation(
 *     singular = "@count country",
 *     plural = "@count countries",
 *   ),
 *   handlers = {
 *     "view_builder" = "Drupal\Core\Entity\EntityViewBuilder",
 *     "views_data" = "Drupal\views\EntityViewsData",
 *     "list_builder" = "Drupal\xxx\Controller\CountryListBuilder",
 *     "form" = {
 *       "add" = "Drupal\xxx\Form\Country\CountryForm",
 *       "edit" = "Drupal\xxx\Form\Country\CountryForm",
 *       "delete" = "Drupal\xxx\Form\Country\CountryDeleteForm",
 *     },
 *     "translation" = "Drupal\content_translation\ContentTranslationHandler",
 *     "access" = "Drupal\Core\Entity\EntityAccessControlHandler",
 *   },
 *   base_table = "xxx_countries",
 *   data_table = "xxx_countries_field_data",
 *   admin_permission = "edit country",
 *   fieldable = FALSE,
 *   translatable = TRUE,
 *   internal = TRUE,
 *   entity_keys = {
 *     "id" = "country_id",
 *     "label" = "name",
 *     "langcode" = "langcode",
 *   },
 *   links = {
 *     "canonical" = "/admin/config/xxx/countries/{country}",
 *     "edit-form" = "/admin/config/xxx/countries/{country}/edit",
 *     "delete-form" = "/admin/config/xxx/countries/{country}/delete",
 *     "collection" = "/admin/config/xxx/countries/list",
 *   },
 *   common_reference_target = TRUE,
 * )

Answer 1

There is only one hit searching t('@label':

/core/lib/Drupal/Core/Entity/Plugin/EntityReferenceSelection/DefaultSelection.php:238

  $fields[$field_name] = $this->t('@label', ['@label' => $field_definition->getLabel()]);

So this is a missing label in the field definition when building the config form to configure the reference field selection.