If you are having this issue on your end, most likely you have enabled Lightbox for Videos through Theme Options > Portfolio > Portfolio Single.
Call to undefined function WP_Filesystem()
Type: PHP Fatal error Line: 115
This issue can be caused by the server configuration.
However, there is a simple solution to this.
All you should do is paste the code snippet below in theme_name/framework/modules/portfolio/portfolio-functions.php file just below the opening PHP tag:
require_once(ABSPATH . 'wp-admin/includes/file.php');
OR you can just disable Lightbox for Video option and that should do the job.