Adding a translation that is not based on the main language will result in

ContextErrorException
HTTP 500 Internal Server Error
Notice: Trying to get property 'languageCode' of non-object

Steps to reproduce
1. Clean installation of eZ Platform EE v2.5.25
2. Log in to the backoffice as the admin user and create a new language: German: ger-DE.
3. Create and publish a test Landing Page in English.
4. For the newly created Landing Page add a German translation, but in the modal, don't select "English" as a base translation.

Result
An exception is being thrown

Expected result
The translation is created normally.