Cannot assign null to property Typecho\Feed::$subTitle of type string

TypeError: Cannot assign null to property Typecho\Feed::$subTitle of type string in /data/www.qiyichao.cn/var/Typecho/Feed.php:143 Stack trace: #0 /data/www.qiyichao.cn/var/Widget/Feed.php(105): Typecho\Feed->setSubTitle() #1 /data/www.qiyichao.cn/var/Typecho/Widget.php(155): Widget\Feed->execute() #2 /data/www.qiyichao.cn/var/Typecho/Router.php(85): Typecho\Widget::widget() #3 /data/www.qiyichao.cn/index.php(23): Typecho\Router::dispatch() #4 {main}