Method getTranslatedField in the class Ibexa\Core\Helper\TranslationHelper should return Field|null according to description, but the method sometimes returns void. You should add "return null" in the end or throw and exception.

Another method getTranslatedFieldValue from the class Ibexa\Core\MVC\Symfony\Templating\Twig\Extension\ContentExtension triggers the method that I mentioned above, but it doesn't check the outcome and than call method "value". It getTranslatedField can also return null, you can't call method value on null. Please add validation. This is only one of the cases that I found. The same issue is with methid 
isFieldEmpty