Blocked user email produces error with Symfony Mailer

Problem/Motivation

When a user attempting to login exceeds the login-attempt threshold, the system blocks the user and an email is sent out. Currently, the last step is producing an exception.

The website encountered an unexpected error. Please try again later.
TypeError: Drupal\symfony_mailer\MailerHelper::parseAddress(): Argument #2 ($langcode) must be of type ?string, Drupal\Core\Language\Language given, called in /app/web/modules/contrib/symfony_mailer/modules/symfony_mailer_bc/src/Plugin/EmailBuilder/LegacyEmailBuilder.php on line 91 in Drupal\symfony_mailer\MailerHelper->parseAddress() (line 78 of modules/contrib/symfony_mailer/src/MailerHelper.php).
Drupal\symfony_mailer\MailerHelper->parseAddress('abc@123.com', Object) (Line: 91)
Drupal\symfony_mailer_bc\Plugin\EmailBuilder\LegacyEmailBuilder->build(Object)
call_user_func(Array, Object) (Line: 460)
Drupal\symfony_mailer\Email->process() (Line: 164)
Drupal\symfony_mailer\Mailer->doSend(Object) (Line: 139)
Drupal\symfony_mailer\Mailer->Drupal\symfony_mailer\{closure}() (Line: 564)
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object) (Line: 148)
Drupal\symfony_mailer\Mailer->send(Object) (Line: 278)
Drupal\symfony_mailer\Email->send() (Line: 97)
Drupal\symfony_mailer_bc\MailManagerReplacement->mail('login_security', 'block_user_notify', 'abc@123.com', Object, Array, 'no-reply@mailer.example.com', 1) (Line: 392)
login_user_block_user_name(Array, Object) (Line: 192)
login_security_validate(Array, Object)
call_user_func_array('login_security_validate', Array) (Line: 82)
Drupal\Core\Form\FormValidator->executeValidateHandlers(Array, Object) (Line: 275)
Drupal\Core\Form\FormValidator->doValidateForm(Array, Object, 'user_login_form') (Line: 118)
Drupal\Core\Form\FormValidator->validateForm('user_login_form', Array, Object) (Line: 588)
Drupal\Core\Form\FormBuilder->processForm('user_login_form', Array, Object) (Line: 320)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object) (Line: 73)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
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: 50)
Drupal\ban\BanMiddleware->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: 709)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

Steps to reproduce

Install Login Security and Symfony Mailer modules.
Configure the Login Security threshold.
Create a new user and activate.
In a separate browser or incognito, produce failed logins for the new user to trigger the account blocking.
Observe the exception shown above.

Proposed resolution

The issue is that a language object is passed instead of a language code in login_security.module:392.

The reason I am reporting this here, instead of Symfony Mailer, is because the core MailManagerInterface (Drupal\Core\Mail\MailManagerInterface) specifies the language argument should be a string.

Remaining tasks

Update the code.

User interface changes

n/a

API changes

n/a

Data model changes

n/a