The editor module assumes that a fieldable entity is also revisionable

The editor_entity_update() function gets an EntityInterface, and checks if it is a FieldableEntityInterface. After that it tries to call getRevisionId(), which will succeed in case of the content entities, since content entities are revisionable and fieldable at the same time. However, if a custom entity type (that is not a subclass of content entity) gets saved and it happens to be fieldable but not revisionable then the editor module will trigger a fatal error.