Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

core/modules/options/src/Plugin/Field/FieldType/ListItemBase.php

@@ -71,7 +71,9 @@ abstract class ListItemBase extends FieldItemBase implements OptionsProviderInte
    * {@inheritdoc}
    */
    public static function generateSampleValue(FieldDefinitionInterface $field_definition) {
-     // @todo Implement this once https://www.drupal.org/node/2238085 lands.
+     $allowed_options = options_allowed_values($field_definition->getFieldStorageDefinition());
+     $values['value'] = array_rand($allowed_options);
+     return $values;
    }
 
   /**

core/modules/options/src/Tests/OptionsFieldTest.php

@@ -80,6 +80,7 @@ class OptionsFieldTest extends OptionsFieldUnitTestBase {
       'field_name' => $this->fieldName,
       'entity_type' => 'entity_test',
       'bundle' => 'entity_test',
+      'required' => TRUE,
     ))->save();
     entity_get_form_display('entity_test', 'entity_test', 'default')
       ->setComponent($this->fieldName, array(