Why am I getting next error: Trying to get property of non-object? laravel

class Article extends Model
{       
    protected $fillable = ['category_id','author_id','title','img','description','text'];

    public function user()
    {
        return $this->belongsTo('App\User');
    }    
    public function article_category()
    {
        return $this->belongsTo('App\ArticleCategory');
    }
}

class ArticleCategory extends Model
{    
    protected $table = 'article_categories';    
    protected $fillable = ['name','hide'];    
    public function article()
    {
        return $this->hasMany('App\Article');
    }
}

User model
    public function article()
        {
            return $this->hasMany('App\Article');
        }

Why am I getting error when trying to get author name and article category name?

Trying to get it in view this way: $article->user->name $article->article_category->name

Can't understand the reason :c

via Batmannn