Laravel 5.3报错：`Creating default object from empty value`

这是代码：

ActivationService.php

<code>    <?php namespace App;


    use Illuminate\Mail\Mailer;
    use Illuminate\Mail\Message;

    class ActivationService
    {

        protected $mailer;

        protected $activationRepo;

        protected $resendAfter = 24;

        public function __construct(Mailer $mailer, ActivationRepository $activationRepo)
        {
            $this->mailer = $mailer;
            $this->activationRepo = $activationRepo;
        }

        public function sendActivationMail($user)
        {

            if ($user->activated || !$this->shouldSend($user)) {
                return;
            }

            $token = $this->activationRepo->createActivation($user);

            $link = route('user.activate', $token);
            $message = sprintf('Activate account <a href="%s">%s</a>', $link, $link);

            $this->mailer->raw($message, function (Message $m) use ($user) {
                $m->to($user->email)->subject('Activation mail');
            });


        }

        public function activateUser($token)
        {
            $activation = $this->activationRepo->getActivationByToken($token);

            if ($activation === null) {
                return null;
            }

            $user = User::find($activation->user_id);
            //下面这行是53行.
            $user->activated = true;

            $user->save();

            $this->activationRepo->deleteActivation($token);

            return $user;

        }

        private function shouldSend($user)
        {
            $activation = $this->activationRepo->getActivation($user);
            return $activation === null || strtotime($activation->created_at) + 60 * 60 * $this->resendAfter </code>

错误信息:

<code>    ErrorException in ActivationService.php line 53:
    Creating default object from empty value</code>

53行的位置在代码中有注释，代码是$user->activated = true;

问题：
上面的代码如何解决这个错误？