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Steps to repeat:
1. Create a node of a Content Type without translations enabled.
2. Enable translations for that Content Type under Administration > Configuration > Regional and language > Content language settings
3. Edit the previously created node.
4. Under the "Translations" tab, the following error appears:
5. Add a new node of that Content Type with translations enabled and the error message does not appear.
Notice: Undefined property: Drupal\node\Plugin\Core\Entity\Node::$translation in translation_entity_overview() (line 99 of core/modules/translation_entity/translation_entity.pages.inc).