From forums:
https://forums.couchbase.com/t/net-2-7-x-nullreferenceexception-encountered-on-getdocumentfromreplicaasync-t/32513
Possible because not set in HandleException:
https://github.com/couchbase/couchbase-net-client/blob/5a33ff12076ab5ec6be99ed454816652a6c0f1b7/Src/Couchbase/IO/Services/IOServiceBase.cs#L262