Issue #3387280: Site setting with just one field fails if once of the base field using BaseFieldOverride

src/SiteSettingsLoader.php

@@ -3,6 +3,7 @@
 namespace Drupal\site_settings;
 
 use Drupal\Core\Entity\EntityTypeManagerInterface;
+use Drupal\Core\Field\Entity\BaseFieldOverride;
 use Drupal\Core\Field\FieldItemListInterface;
 use Drupal\Core\Language\LanguageManagerInterface;
 use Drupal\Core\Url;
@@ -170,7 +171,7 @@ class SiteSettingsLoader implements SiteSettingsLoaderInterface {
       $field_definition = $field->getFieldDefinition();
 
       // Exclude fields on the object that are base config.
-      if (!method_exists(get_class($field_definition), 'isBaseField') || !$field_definition->isBaseField()) {
+      if ((!method_exists(get_class($field_definition), 'isBaseField') || !$field_definition->isBaseField()) && !($field_definition instanceof BaseFieldOverride)) {
 
         if (($value = $this->getValue($field)) || $field_definition->getType() == 'boolean') {
           $build[$key] = $value;