Scenario: I'm validating an entity like this

$validator->validate($device, [
    new UniqueEntity([
        'fields' => ['accountId', 'tenantUuid'],
        'em' => $provisioningEm
    ])
]);

If accountId happens to be null, UniqueEntityValidator will now exclude it from the constraint list (since commit 1e3421d ), failing later on because $criteria[$fields[0]] is an invalid array access: fields[0] is the string "accountId", which is not in $criteria.
Reverting lines 88-94 to the version before the commit linked above fixes the issue.