encode method Null safety

Implementation

@override
void encode(KeyedArchive object) {
  super.encode(object);

  if (content == null) {
    throw ArgumentError(
      "APIRequestBody must have non-null values for: 'content'.",
    );
  }

  object.encode("description", description);
  object.encode("required", isRequired);
  object.encodeObjectMap("content", content);
}