Steps:

1. Add new language
2. Create new article
3. Go to Translations tab
4. Click button to add new translation
5. Select new language and confirm

Actual result:

There is an error: Cannot check if an unsubmitted form is valid. Call Form::isSubmitted() before Form::isValid(). [in vendor/symfony/form/Form.php:743]

Expected result:

Translation form should be displayed.