添加话题回复时出现Call to ... App\\Models\\Topic::updateReplyCount()

测试增加话题回复时，报错如下图：

应该是触发了教程L02的更新话题回复数的观察器,不知道为何没有调用App\Models\Reply里的updateReplyCount，而是去报这个错
Call to undefined method App\Models\Topic::updateReplyCount()：
ReplyObserver.php

<?php

namespace App\Observers;

use App\Models\Reply;
use App\Notifications\TopicReplied;

class ReplyObserver
{
  //统计回复数
  public function created(Reply $reply) {  
  // 命令行运行迁移时不做这些操作！
  if ( ! app()->runningInConsole()) {
      $reply->topic->updateReplyCount();
      // 通知话题作者有新的评论
      $reply->topic->user->notify(new TopicReplied($reply));
  }
 }
}

Models 里的Reply.php

<?php

namespace App\Models;

class Reply extends Model
{

    protected $fillable = ['content'];

  public function topic()
 {  
     return $this->belongsTo(Topic::class);
  }

  public function user()
 {  
     return $this->belongsTo(User::class);
  }

  public function updateReplyCount()
 {  
      $this->reply_count = $this->replies->count();
      $this->save();
  }
}