TypeError when extending EmailWebformHandler

I was trying to create a custom EmailWebformHandler using an example at the following URL.

In this example the sendMessage()-function has one argument: an array called $message.
However, when I use this example to create my own EmailWebformHandler I get an error in PhpStorm:
Declaration must be compatible with WebformHandlerMessageInterface->sendMessage(webform_submission: \Drupalwebform\WebformSubmissionInterface, message: array)

Because the official example contained the code this way, i tried it, but i couldn't add my EmailWebformHandler to the webform, so I tried solving this problem by adding the argument that was missing: WebformSubmissionInterface $webform_submission

This way I could add my custom EmailWebformHandler and change the needed settings, but when I tried sending my form, I got the following error:

TypeError: Argument 1 passed to Drupal\webform\Plugin\WebformHandler\EmailWebformHandler::sendMessage() must implement interface Drupal\webform\WebformSubmissionInterface, array given, called in /Users/michaelsoetaert/Webdev/sites/tvh/git/www.tvh-d8.com/modules/custom/tvh_webform_handlers/src/Plugin/WebformHandler/SendToFriendEmailWebformHandler.php on line 26 in Drupal\webform\Plugin\WebformHandler\EmailWebformHandler->sendMessage() (line 754 of modules/contrib/webform/src/Plugin/WebformHandler/EmailWebformHandler.php).
Drupal\webform\Plugin\WebformHandler\EmailWebformHandler->sendMessage(Array) (Line: 26)
Drupal\tvh_webform_handlers\Plugin\WebformHandler\SendToFriendEmailWebformHandler->sendMessage(Object, Array) (Line: 529)
Drupal\webform\Plugin\WebformHandler\EmailWebformHandler->postSave(Object, , NULL) (Line: 1639)
Drupal\webform\Entity\Webform->invokeHandlers('postSave', Object, , NULL) (Line: 751)
Drupal\webform\WebformSubmissionStorage->invokeWebformHandlers('postSave', Object, ) (Line: 701)
Drupal\webform\WebformSubmissionStorage->doPostSave(Object, ) (Line: 395)
Drupal\Core\Entity\EntityStorageBase->save(Object) (Line: 768)
Drupal\Core\Entity\Sql\SqlContentEntityStorage->save(Object) (Line: 364)
Drupal\Core\Entity\Entity->save() (Line: 681)
Drupal\webform\Entity\WebformSubmission->save() (Line: 944)
Drupal\webform\WebformSubmissionForm->save(Array, Object)
call_user_func_array(Array, Array) (Line: 111)
Drupal\Core\Form\FormSubmitter->executeSubmitHandlers(Array, Object) (Line: 51)
Drupal\Core\Form\FormSubmitter->doSubmitForm(Array, Object) (Line: 585)
Drupal\Core\Form\FormBuilder->processForm('webform_submission_send_to_friend_form', Array, Object) (Line: 314)
Drupal\Core\Form\FormBuilder->buildForm('webform_submission_send_to_friend_form', Object) (Line: 48)
Drupal\Core\Entity\EntityFormBuilder->getForm(Object, 'default') (Line: 823)
Drupal\webform\Entity\Webform->getSubmissionForm() (Line: 82)
Drupal\webform\Controller\WebformController->addForm(Object, Object)
call_user_func_array(Array, Array) (Line: 123)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}() (Line: 574)
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}()
call_user_func_array(Object, Array) (Line: 144)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1) (Line: 64)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1) (Line: 57)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1) (Line: 99)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1) (Line: 78)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1) (Line: 47)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1) (Line: 50)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1) (Line: 23)
Stack\StackedHttpKernel->handle(Object, 1, 1) (Line: 656)
Drupal\Core\DrupalKernel->handle(Object) (Line: 19)

The error is thrown because my first argument is an array, but they expect WebformSubmissionInterface. However in my code, my first argument actually is WebformSubmissionInterface:

public function sendMessage(WebformSubmissionInterface $webform_submission, array $message) {
    kint($message);
    parent::sendMessage($message);
  }

I'm guessing that this is a bug, since both the code in the official example and the code found in EmailWebformHandler.php don't work.