Dart: How to Encode Decode animated webp files?

Question

I am trying to decode animated webp image file and encode it's frames to animated webp image file in Dart (cli)

Dart Code:

import 'dart:io';
import 'package:image/image.dart';

void main() {
    final image = 'asset/test.webp';

    File bytes = File(image);
    List<int> list = bytes.readAsBytesSync();

    var anim = decodeWebPAnimation(list)!;

    var obj = WebPEncoder();

    for (var frame in anim) {
      obj.addFrame(frame, duration: 1);
    }

    var finished = obj.finish();

    print(finished); // [] empty, no frames get added
    // final encodedAnim = obj.encodeAnimation(); 

    File('asset/output.webp').writeAsBytesSync(finished!,
      flush: true); // pass something like encodeWebp(input)
}

pubspec.yaml

dependencies:
    image: ^3.1.3

input file: https://raw.githubusercontent.com/WhatsApp/stickers/main/Android/app/src/main/assets/2/07_OK.webp

Output: There is zero frame after calling finish(), so the output image file is invalid. i.e. file size: 0 byte

Ref of Dart APIs:

WebpEncoder: https://pub.dev/documentation/image/latest/image/WebPEncoder-class.html

DecodeWebpAnimation: https://pub.dev/documentation/image/latest/image/decodeWebPAnimation.html

What went wrong? How to fix this? Thank You!