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

This is the code:

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 < time();
        }
    
    }</code>

Error message:

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

The position of line 53 is commented in the code, the code is $user->activated = true;

Question:
How does the above code solve this error?