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

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