Deprecation warning

Please see the following deprecation warning that occurs on our site:

E_DEPRECATED: Creation of dynamic property CssUtilities::$url_for_fix is deprecated (/var/www/ultimatehackingkeyboard.com/wordpress/wp-content/plugins/wp-fastest-cache/inc/css-utilities.php:412) CssUtilities::fixPathsInCssContent(/var/www/ultimatehackingkeyboard.com/wordpress/wp-content/plugins/wp-fastest-cache/inc/css-utilities.php:376) CssUtilities::minify(/var/www/ultimatehackingkeyboard.com/wordpress/wp-content/plugins/wp-fastest-cache/inc/css-utilities.php:164) CssUtilities::create_content(/var/www/ultimatehackingkeyboard.com/wordpress/wp-content/plugins/wp-fastest-cache/inc/css-utilities.php:118) CssUtilities::combineCss(/var/www/ultimatehackingkeyboard.com/wordpress/wp-content/plugins/wp-fastest-cache/inc/cache.php:852) WpFastestCacheCreateCache::callback(:) ob_end_flush(/var/www/ultimatehackingkeyboard.com/wordpress/wp-includes/functions.php:5373) wp_ob_end_flush_all(/var/www/ultimatehackingkeyboard.com/wordpress/wp-includes/class-wp-hook.php:324) WP_Hook::apply_filters(/var/www/ultimatehackingkeyboard.com/wordpress/wp-includes/class-wp-hook.php:348) WP_Hook::do_action(/var/www/ultimatehackingkeyboard.com/wordpress/wp-includes/plugin.php:517) do_action(/var/www/ultimatehackingkeyboard.com/wordpress/wp-includes/load.php:1260) shutdown_action_hook(:)