parseKnownPayload function
Parse payload and return KnownPayload or throws error
Implementation
KnownPayload? parseKnownPayload(String payload) {
final res = ntParseKnownPayload(payload: payload);
if (res == null) return null;
final decoded = jsonDecode(res);
if (decoded == null) return null;
return KnownPayload.fromJson(decoded);
}