为什么通过user模型可以访问相关联的article模型的属性，但是通过article模型访问user会报错“Attempt to read property "nickname" on null”

小白求教：
在larval9框架下建立了两个模型user和article，两个模型之间是一对多的关系，通过user模型可以访问article模型，但是通过article模型访问user模型就会出错

//user模型
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\MassAssignmentException;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Notifications\Notifiable;
use Illuminate\Support\Facades\DB;
use Laravel\Sanctum\HasApiTokens;
class User extends Authenticatable
{
    use HasApiTokens, HasFactory, Notifiable;
    protected $table='users';
    protected $fillable = [
        'name',
        'email',
        'password',
        'telephone',
        'wx',
        'thumb',
        'description',
        'age',
        'school',
        'nickname'
    ];
    protected $hidden = [
        'password',
        'remember_token',
    ];
    protected $casts = [
        'email_verified_at' => 'datetime',
    ];
    function ArticleBy(){
        return $this->hasMany(Article::class);
    }
}
//article模型
<?php
namespace App\Models;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Database\Eloquent\Model;
class Article extends Model
{
  use HasFactory;
  protected $table='articles';
  protected $fillable=[
  'Author',
  'context',
  'mark',
  'count',
  'like',
  'context_alter'
  ];
  function UserBy(){
  return $this->belongsTo(User::class);
 }

}

//控制器方法
public function article_detail($id){
  $article=Article::find($id);
  return view('admin.article_detail')->with('article',$article);
}

//前端显示
<div class="article_title">标题：{{$article->title}}</div>
<div class="article_author">作者：{{$article->UserBy->nickname}}</div>
 <div class="article_like">点赞数：{{$article->like}}</div>
 <div class="article_count">搜索数：{{$article->count}}</div>
 <div class="article_cerated">发布时间：{{$article->created_at}}</div>

user表

article表

然后报错提示的SQL信息是“select * from articles where articles.id = ? limit 1”