Route::get('test', function () {
    $data = \app\entity\Orders::find(84);
    dump($data);
});

Cannot assign null to property app\entity\Orders::$mstatus of type string

<?php
// +----------------------------------------------------------------------
// | 商户模型
// +----------------------------------------------------------------------
namespace app\model;

use think\Model;

class Merchants extends Model
{
}

<?php
// +----------------------------------------------------------------------
// | 订单模型
// +----------------------------------------------------------------------
namespace app\model;

use think\Model;
use think\model\relation\BelongsTo;

class Orders extends Model
{

    /**
     * 关联商户
     */
    public function merchant(): BelongsTo
    {
        return $this->belongsTo(Merchants::class, 'merchant_id', 'id');
    }

}

<?php
// +----------------------------------------------------------------------
// | 商户视图模型
// +----------------------------------------------------------------------
namespace app\entity;

use think\model\View;

class Merchants extends View
{


    public string $id;
    public string $status;

}

<?php
// +----------------------------------------------------------------------
// | 订单视图模型
// +----------------------------------------------------------------------
namespace app\entity;

use think\model\View;

class Orders extends View
{

    public string $id;
    public string $merchant_id;
    public string $amount;
    public string $mstatus;

    public function getOptions():array
    {
        return [
            //'autoMapping' => ['merchant']
            'viewMapping' => ['mstatus' => 'merchant->status']
        ];
    }
}