Warning: Undefined array key "validate_subject" in SimplenewsEmailBuilder->import()

Problem/Motivation

Warning: Undefined array key "validate_subject" in Drupal\symfony_mailer_bc\Plugin\EmailBuilder\SimplenewsEmailBuilder->import() (line 65 of modules/contrib/symfony_mailer/modules/symfony_mailer_bc/src/Plugin/EmailBuilder/SimplenewsEmailBuilder.php).
Drupal\symfony_mailer_bc\Plugin\EmailBuilder\SimplenewsEmailBuilder->import() (Line: 137)
Drupal\symfony_mailer\Processor\EmailBuilderManager->import('simplenews') (Line: 147)
Drupal\symfony_mailer\Processor\EmailBuilderManager->importAll() (Line: 95)
Drupal\symfony_mailer\Controller\SymfonyMailerController->importAll()
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 124)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array) (Line: 97)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 158)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 80)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 58)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 106)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 85)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 48)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 51)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 708)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

And then:

Warning: Undefined array key "validate_body" in Drupal\symfony_mailer_bc\Plugin\EmailBuilder\SimplenewsEmailBuilder->import() (line 66 of modules/contrib/symfony_mailer/modules/symfony_mailer_bc/src/Plugin/EmailBuilder/SimplenewsEmailBuilder.php).
...

Steps to reproduce

Install drupal/simplenews version 3.0.0-beta2.
Go to /admin/config/system/mailer/import and click Import for Simplenews subscriber settings (simplenews).