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

這是代碼：

ActivationService.php

    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 %s', $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);            //下面這行是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 < time();        }        }

錯誤資訊:

    ErrorException in ActivationService.php line 53:    Creating default object from empty value

53行的位置在代碼中有注釋，代碼是$user->activated = true;

問題：
上面的代碼如何解決這個錯誤？

