Symfony version(s) affected: 4.4.11

Description

When a form triggers some error, the code flow goes that way:
https://github.com/symfony/symfony/blob/v4.4.11/src/Symfony/Component/Validator/Violation/ConstraintViolationBuilder.php#L161-L165
https://github.com/symfony/symfony/blob/v4.4.11/src/Symfony/Component/Translation/DataCollectorTranslator.php#L54-L60
https://github.com/symfony/symfony/blob/v4.4.11/src/Symfony/Component/Translation/Translator.php#L224
https://github.com/symfony/symfony/blob/v4.4.11/src/Symfony/Component/Translation/Translator.php#L284-L297

How to reproduce

Inside my tests there are errors like that:

TypeError: Argument 1 passed to Symfony\Component\Translation\Translator::initializeCacheCatalogue() must be of the type string, null given, called in vendor/symfony/symfony/src/Symfony/Component/Translation/Translator.php on line 317

vendor/symfony/symfony/src/Symfony/Component/Translation/Translator.php:338
vendor/symfony/symfony/src/Symfony/Component/Translation/Translator.php:317
vendor/symfony/symfony/src/Symfony/Component/Translation/Translator.php:293
vendor/symfony/symfony/src/Symfony/Component/Translation/Translator.php:224
vendor/symfony/symfony/src/Symfony/Component/Translation/DataCollectorTranslator.php:56
vendor/symfony/symfony/src/Symfony/Component/Validator/Violation/ConstraintViolationBuilder.php:164
vendor/symfony/symfony/src/Symfony/Component/Validator/Constraints/NotBlankValidator.php:45
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:767
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:609
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:520
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:315
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:138
vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Validator/Constraints/FormValidator.php:104
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:767
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:492
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:315
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php:138
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/RecursiveValidator.php:93
vendor/symfony/symfony/src/Symfony/Component/Validator/Validator/TraceableValidator.php:66
vendor/symfony/symfony/src/Symfony/Component/Form/Extension/Validator/EventListener/ValidationListener.php:50
vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php:264
vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php:239
vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/EventDispatcher.php:73
vendor/symfony/symfony/src/Symfony/Component/EventDispatcher/ImmutableEventDispatcher.php:44
vendor/symfony/symfony/src/Symfony/Component/Form/Form.php:671
vendor/symfony/symfony/src/Symfony/Component/Form/Extension/HttpFoundation/HttpFoundationRequestHandler.php:109
vendor/symfony/symfony/src/Symfony/Component/Form/Form.php:493

Possible Solution
Allow nullable locale? Or manual how to properly set a locale in tests/CLI/whatever?

Additional context
With Symfony 3.4.35 there are no errors like these.

I also tried to add \Locale::setDefault('en'); inside setUpBeforeClass() method, but didn't work.