500 Internal Server Error

Undefined array key "HTTP_ACCEPT_LANGUAGE" (2)

#0 /var/www/html/app/controllers/HomeController.php(54): flight\Engine->handleError()
#1 /var/www/html/app/controllers/HomeController.php(88): HomeController::getUserLang()
#2 [internal function]: HomeController::cookiePolicy()
#3 /var/www/html/vendor/mikecao/flight/flight/core/Dispatcher.php(162): call_user_func_array()
#4 /var/www/html/vendor/mikecao/flight/flight/core/Dispatcher.php(145): flight\core\Dispatcher::callFunction()
#5 /var/www/html/vendor/mikecao/flight/flight/Engine.php(341): flight\core\Dispatcher::execute()
#6 /var/www/html/vendor/mikecao/flight/flight/core/Dispatcher.php(198): flight\Engine->_start()
#7 /var/www/html/vendor/mikecao/flight/flight/core/Dispatcher.php(144): flight\core\Dispatcher::invokeMethod()
#8 /var/www/html/vendor/mikecao/flight/flight/core/Dispatcher.php(49): flight\core\Dispatcher::execute()
#9 /var/www/html/vendor/mikecao/flight/flight/Engine.php(92): flight\core\Dispatcher->run()
#10 /var/www/html/vendor/mikecao/flight/flight/core/Dispatcher.php(198): flight\Engine->__call()
#11 /var/www/html/vendor/mikecao/flight/flight/Flight.php(77): flight\core\Dispatcher::invokeMethod()
#12 /var/www/html/public/index.php(17): Flight::__callStatic()
#13 {main}