Quote from the licensee
(FbxSkeletalMeshImport.cpp inside "// merge bones to the selected skeleton"): this happens because in UnFbx::FFbxImporter::SetupAnimationDataFromMesh, the function FindFBXMeshesByBone returns a nullptr but nothing checks for it, so RecursiveBuildSkeleton and IsUnrealBone will use a nullptr.
Reimport a skeletal mesh with some change in the skeleton
RESULT
Editor can crash depending on the skeleton change
EXPECTED
Editor handle nicely the case and do not crash
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-43865 in the post.