toJson method Null safety
Implementation
String toJson() => jsonEncode(<String,String?>{
"ownershipId" : String.fromCharCodes(ownershipId),
"destination" : destination.toJson(),
"about" : about,
"reward" : reward,
"transactionId" : String.fromCharCodes(transactionId!),
"expiry" : expiry?.millisecondsSinceEpoch.toString()
});