Trying to access array offset on value of type null

	{
		$classid = input('get.classid');
		// 当前栏目信息
		$newsClass = Db::name('nav')->field('title,url,parentid')->where("id=$classid")->select();
			
		$this->assign('newsClass',$newsClass);
		// 父级栏目信息
		$parentid = $newsClass[0]['parentid'];
		$parentClass = Db::name('nav')->field('title,url,parentid')->where("id=$parentid")->find();
		$parentName = $parentClass['title'];
		$this->assign('parentName',$parentName);

		// 新闻分页
		$where['query']['classid'] = $classid;
		$newsList = Db::name('article')->where(['status' => 1,'classid' => $classid])->order('create_time desc,istop desc,orderid asc,create_time desc')->paginate(10,'',$where);
		
		$this->assign('newsList',$newsList);
		return $this->fetch();
	}