tp6 模型 Call to a member function allowField() on null

$rs7 = (new static())->setSuffix($setSuffix_7d)->find($postdata['id']);
if(!empty($rs7)){//如果上面找不到，就不能更新
   $rs7->allowField(['label','gid','phone','username','wx''])->save($postdata);//7天表 //模型出错，自动抛出,不用判断
}