Mailer test fails with Call to a member function getSubject() on null - symfony

Question

In a Symfony 5.0.2 project a test of the new Mailer fails with
Error: Call to a member function getSubject() on null
The email service and test are based on symfonycast tutorials.
Adding var_dump($email); in the service immediately after $email = ...; shows object(Symfony\Bridge\Twig\Mime\TemplatedEmail)#24 (11) {..., which says there is a real object created in the service.
services.yaml:
App\Services\EmailerService:
$mailer: '#mailer'
$senderAddress: '%app.sender_address%'
Service:
use Symfony\Bridge\Twig\Mime\TemplatedEmail;
class EmailerService
{
private $mailer;
private $sender;
public function __construct($mailer, $senderAddress)
{
$this->mailer = $mailer;
$this->sender = $senderAddress;
}
public function appMailer($mailParams)
{
$email = (new TemplatedEmail())
->from($this->sender)
->to($mailParams['recipient'])
->subject($mailParams['subject'])
->htmlTemplate($mailParams['view'])
->context($mailParams['context']);
$this->mailer->send($email);
}
}
Test:
use App\Services\EmailerService;
use PHPUnit\Framework\TestCase;
use Symfony\Component\Mailer\MailerInterface;
class MailerTest extends TestCase
{
public function testSimpleMessage()
{
$symfonyMailer = $this->createMock(MailerInterface::class);
$symfonyMailer->expects($this->once())
->method('send');
$mailer = new EmailerService($symfonyMailer, 'admin#bogus.info', 'admin#bogus.info');
$mailParams = [
'view' => 'Email/non_user_forgotten_password.html.twig',
'context' => ['supportEmail' => 'admin#bogus.info'],
'recipient' => 'bborko#bogus.info',
'subject' => 'Test message',
];
$email = $mailer->appMailer($mailParams);
$this->assertSame('Test message', $email->getSubject());
}
}

Answer 1

appMailer() must return a TemplatedEmail object so you can call getSubject() on it. Currently it is returning nothing. Change it to:
public function appMailer($mailParams)
{
$email = (new TemplatedEmail())
->from($this->sender)
->to($mailParams['recipient'])
->subject($mailParams['subject'])
->htmlTemplate($mailParams['view'])
->context($mailParams['context']);
$this->mailer->send($email);
return $email; // I added this line.
}