I recon this question is 2 years old. However the reason as to why this assertion fails hasn’t changed to date (version 4.13.1 of the engine). It might help a fellow reader.
Anywho, this code simply crashes because one of the pairs the author tries to address here - particularly PathQueue[Key]["PathList"] - does not exist: either Key or "PathList".
Always ensure the pair exists by checking with, in this example, PathQueue.Contains(Key) and PathQueue[Key].Contains(FString(TEXT("PathList"))) (assuming PathQueue is a TMap<..., TMap>.