Symfony在发送消息时调用null上的成员函数format（）

Question

我试图在坚持之后发送消息。当我使用常规方式时，它可以正常工作

addReservatieAction：

    public function addReservatieAction(Request $request, \Swift_Mailer $mailer)
{  //the rest of the code is also here
        $message = (new \Swift_Message('Hello Email'))
            ->setFrom('example@live.nl')
            ->setTo($email)
            ->setBody("$success,<br>Email:$email,<br>Name:$name,<br>
                        StartDate:$start_date,<br>EndDate:$end_date", 'text/html');

        $mailer->send($message);

    }

但我想为这样的邮件制作另一个功能：

addReservatieAction：

     public function addReservatieAction(Request $request, \Swift_Mailer $mailer)
    {

        $reservatie = new reservatie();

        $form = $this->createForm(reservatieType::class, $reservatie);
       $form->handleRequest($request);

        if ($form->isSubmitted() && $form->isValid()) {

            $message=$this->sendEmail();
            //add flashbag is room is booked
            $mailer->send($message);

            $url = $this->generateUrl('index_reservatie');

            return $this->redirect($url);
        }

  public function sendEmail (){

        $reservatie = new reservatie();

        $start_date = $reservatie->getDateIn();
        $start_date = $start_date->format('d-m-y');

        $end_date = $reservatie->getDateOut();
        $end_date = $end_date->format('d-m-y');

        $message = (new \Swift_Message('Hello Email'))
            ->setFrom('example@live.nl')
            ->setTo($email)
            ->setBody(" StartDate:$start_date,<br>EndDate:$end_date", 'text/html');

        return $message;

    }

但问题是它一直给我这个错误：

  

在null

上调用成员函数format（）

我在约会时使用它并非空。