Warning: Undefined global variable $_SESSION in /var/www/html/classes/Navigation.php on line 20

Fatal error: Uncaught TypeError: array_key_exists(): Argument #2 ($array) must be of type array, null given in /var/www/html/classes/Navigation.php:20 Stack trace: #0 /var/www/html/classes/Navigation.php(20): array_key_exists('session_login_u...', NULL) #1 /var/www/html/classes/Header.php(18): lts\classes\Navigation::create('<!doctype html>...') #2 /var/www/html/berichte/index.php(10): lts\classes\Header::create('\xC3\x9Cbersicht der ...', 'Langbogenturnie...', 'langbogen,engli...') #3 {main} thrown in /var/www/html/classes/Navigation.php on line 20