Backup PHP Fatal Error

While manually running backup from backup page, it displays:

Storing backup…
PHP Fatal error: Call to a member function directoryExists() on a non-object in /var/www/html/admin/modules/backup/functions.inc/class.backup.php on line 423
Whoops\Exception\ErrorException: Call to a member function directoryExists() on a non-object in file /var/www/html/admin/modules/backup/functions.inc/class.backup.php on line 423
Stack trace:

  1. () /var/www/html/admin/modules/backup/functions.inc/class.backup.php:423

I will try fwconsole chown and try it again:

Storing backup…
PHP Fatal error: Call to a member function directoryExists() on a non-object in /var/www/html/admin/modules/backup/functions.inc/class.backup.php on line 423
Whoops\Exception\ErrorException: Call to a member function directoryExists() on a non-object in file /var/www/html/admin/modules/backup/functions.inc/class.backup.php on line 423
Stack trace:

  1. () /var/www/html/admin/modules/backup/functions.inc/class.backup.php:423
    done!

How to fix this?
Thanks.