After opens the preloaded level using Asset Manager, opening another level before unloading the level will crash. Because AssetManager holds FStreamableHandle of the level, garbage collection is not performed correctly.
You can avoid this crash by commenting out UEngine :: VerifyLoadMapWorldCleanup. However, re-opening that level will cause another crash. This is because UE4 tries to use UWorld have many null properties present in memory.
If comment out UEngine::VerifyLoadMapWorldCleanup
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-69214 in the post.