To fix the following notice-level errors:

Notice: Undefined property: Doctrine\DBAL\Connections\MasterSlaveConnection::$_conn in vendor/doctrine/dbal/lib/Doctrine/DBAL/Connections/MasterSlaveConnection.php line 154
Notice: Undefined index: slave in vendor/doctrine/dbal/lib/Doctrine/DBAL/Connections/MasterSlaveConnection.php line 165