error message on my start page

I have an error message on my start page and don't know how to fix it? The error message: Warning: Undefined variable $orderby in /home/httpd/vhosts/kletterlehrerin.ch/httpdocs/wp-content/themes/catch-eve rest-pro/inc/catcheverest-functions.php on line 957 Warning: Undefined variable $catcheverest_category_sliders in /home/httpd/vhosts/kletterlehrerin.ch/httpdocs/wp-content/themes/catch-eve rest-pro/inc/catcheverest-functions.php on line 994