Nem sikerült létrehozni az adatbázis-kapcsolatot! [see the MYSQL Exception error log file]

Fatal error: Uncaught Error: Call to a member function prepare() on null in /var/www/html/Model/Db.php:57 Stack trace: #0 /var/www/html/Model/LoginSettingModel.php(356): Db->query() #1 /var/www/html/Controller/LoginSetting.php(348): LoginSettingModel->setRemoveLogins() #2 /var/www/html/index.php(7): LoginSetting->RemoveOldLogins() #3 {main} thrown in /var/www/html/Model/Db.php on line 57