Fatal error: Uncaught Error: Class "PKP\migration\Migration" not found in C:\xampp\htdocs\ojs\lib\pkp\classes\migration\upgrade\v3_4_0\I7249_UpdateUsersUniqueIndex_v3_1.php:20 Stack trace: #0 {main} thrown in C:\xampp\htdocs\ojs\lib\pkp\classes\migration\upgrade\v3_4_0\I7249_UpdateUsersUniqueIndex_v3_1.php on line 20