In UEngine::TickWorldTravel, in between calls to LoadMap() and LoadMapCompleted() it is possible that an error causes PendingNetGame on the world context to be nulled out (presumably by CancelPending), resulting in a crash.
See description, this is unlikely to occur in the wild (although did for one licensee due to custom connection logic).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-60547 in the post.