Autocomplete field widget error in case of BaseField

I know it has been a long time since you posted this issue, but could you provide steps how to reproduce the error?

Hi. As I remember there was a problem if the field was a base field (defined in the entity class) of the custom entity type.

Steps to reproduce:
1. Create a custom entity type. It can be generated via drush or drupal console.
2. Add the country field to this custom entity type. (baseFieldDefinitions method)

$fields['residence_country'] = BaseFieldDefinition::create('country')
      ->setLabel(t('Country of residence'))
      ->setRevisionable(TRUE)
      ->setDisplayConfigurable('form', TRUE)
      ->setDisplayConfigurable('view', TRUE)
      ->setRequired(TRUE);

3. Go to the entity create/edit form and try to add a value to the Country field. You will see the error.

Note: I'm not sure whether the issue is still relevant or not.

I tested this with a custom entity and Country as a base field.

The problem does not occur anymore.