toJson method Null safety

Implementation

String toJson() {
  return "{"
      "\"id\":\"$id\","
      "\"localCreated\":\"$localCreated\","
      "\"localUpdated\":\"$localUpdated\","
      "\"apiBaseUrl\":\"$apiBaseUrl\","
      "\"keyHandle\":\"$keyHandle\","
      "\"state\":\"$state\","
      "\"created\":\"$created\","
      "\"updated\":\"$updated\","
      "\"tenant\":${tenant.toJson()},"
      "\"realm\":${realm.toJson()},"
      "\"identity\":${identity.toJson()},"
      "\"theme\":${theme.toJson()}}";
}