StopTransaction.req json messgae without field reason will lead to crash

[MarkJlMa]

Within method StopTransactionReqConverter::fromJson,
data.reason = ReasonHelper.fromString(json["reason"].GetString()) doesn't check if the message contains [reason] memeber.

[c-jimenez]

Thanks, the reason field was not marked as optional in the StopTransactionReq struct.
A fix will come shortly in the develop branch.