Undefined index: HTTP_USER_AGENT

I get the following NOTICE message:
PHP Notice: Undefined index: HTTP_USER_AGENT in ...public_html/wp-content/plugins/wp-fastest-cache/inc/cache.php on line 416

Proposed solution:

public function isMobile(){
    if(isset($_SERVER['HTTP_USER_AGENT']) && preg_match("/.*".$this->getMobileUserAgents().".*/i", $_SERVER['HTTP_USER_AGENT']))
	return true;
    return false;
}

Please note that there are similar NOTICES around lines 130-140.
Similar solution works for me there, too.

https://wordpress.org/plugins/wp-fastest-cache/