This issue is created automatically through a Github pull request on behalf of jappie:
While executing updates on an entity scheduled for update without a change-set, an "Undefined index" notice is raised.
This issue will occur when you manually call $em()->getUnitOfWork()->scheduleForUpdate() on an entity that hasn't changed. The entity will be included in UnitOfWork::$entityUpdates, but because there are no changes, its oid will not be included in UnitOfWork::$entityChangeSets.
I know I'm misusing scheduleForUpdate() a bit in this case, but the notice can easily be prevented with a !empty().