Missing property $parentCollection exception on addChild()-call

[menglisch]

The following code throws an exception:

// $tenantName, $privateKey, $clientId, $thumbprint are provided but left out here

$client = new GraphServiceClient(function () use ($tenantName, $privateKey, $clientId, $thumbprint) {
	$resource = "https://graph.microsoft.com";
	$provider = new AADTokenProvider($tenantName);
	$credentials = new CertificateCredentials($resource, $clientId, $privateKey, $thumbprint, ['https://graph.microsoft.com/.default']);

	return $provider->acquireTokenForClientCertificate($credentials);
});

$user = $client->getUsers()->getByUserPrincipalName('smith@example.com');

/** @var \Office365\Outlook\Events\Event $event */
$event = $user->getEvents()->add();

$start = new DateTimeTimeZone();
$start->DateTime = '2025-10-01';
$start->TimeZone = 'Europe/Berlin';
$event->setStart($start);
$event->setIsAllDay(true);
$event->setBody(new ItemBody(0, 'Test'));

$addedChildQuery = $user->getEvents()->addChild($client); // here is the exception thrown
$addedChildQuery->executeQuery();

But an exception is thrown:

   ErrorException

   Undefined property: Office365\GraphServiceClient::$parentCollection

  at vendor/vgrem/php-spo/src/Runtime/ClientObjectCollection.php:138
    134▕      */
    135▕     public function addChild($clientObject)
    136▕     {
    137▕         $this->data[] = $clientObject;
  ➜ 138▕         if (is_null($clientObject->parentCollection))
    139▕             $clientObject->parentCollection = $this;
    140▕         return $this;
    141▕     }
    142▕

What am I doing wrong?
Does anybody know how to create an event in Office 365 with this library?