Level Instance: failed check in duplication code when embedding

Mentioned in this UDN is the possibility that duplication of some actor fails because their BP as been deleted. 

This is from The Coalition:

"
WorldPartitionLevelHelper :: check(DuplicatedActor) fails often
Hi, as I push on WorldPartition a crash I hit pretty frequently is that this check(DuplicatedActor) in the WorldPartitionLevelHelper fails.

Have there been improvements around this section of the code I could request? or in the short-term is making this an if() block instead of a check your suggestion? I assume it is bad if there are actors in the mapping list that aren't real, but don't know where to harden the code to prevent these from entering.

In one case an actor existed of a particular blueprint class. the Blueprint class was deleted, which caused the World Partition level to throw an error on load saying the blueprint class could not be found. Unfortunately the actor then doesn't show up in the level editor. So it couldn't be removed. Because it couldn't be removed it was still in the mapping list, which would then fail during this duplicate step.

In this case, finding the actor in the External_Actors folder and deleting it seems to have worked, but unlike an old style level, just resaving the map won't clean these actors up."

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