在null上调用成员函数addMessage（），

Question

I am trying to add slim flash message to be accessed with my controller using Slim framework 3 & Twig, But I get the following error :

Call to a member function addMessage() on null, Buzz\Controllers\MailController::$flash in C:\xampp\htdocs\myapp\app\Controllers\MailController.php on line 63

   // bootfile.php
     <?php
      $container['MailController'] = function($container){ return new \Buzz\Controllers\MailController($container); };                                                                                                                                        
       $container['flash'] = function($container){   return new \Slim\Flash\Messages; };
       $container['view'] = function($container){
        $twig = new \Slim\Views\Twig(__DIR__ . '/../pages/views', [  'cache' => false, ]);
        $twig->addExtension(new \Slim\Views\TwigExtension( $container->router,  $container->request->getUri()));
        $view->getEnvironment()->addGlobal('flash', $container->flash);
        return $twig;
     };

    // MailController.php
     public function sendmail($request, $response){
         $sent = mail->send();
         if ($sent) {
                     $this->flash->addMessage('mailsuccess', 'Thank you for contacting');
         }
     }