How can I write a MapMapper for the parts map at WithParts class below as mentioned at Custom Mappers (custom Iterables and Maps)

I already tried to use the skeleton available as Custom Mappers page but I can´t make it work. I get a JsonUnsupportedObjectError (converting object to an encodable object faile; _Map len:2) exception at the toJson() line.

    import 'package:dart_mappable/dart_mappable.dart';
    import 'package:flutter/material.dart';
    
    part 'dart_mappable_mapmapper.mapper.dart';
    
    @MappableClass()
    class Part with PartMappable {
      final String partID;
      late final Offset _value;
    
      Part(this.partID, Offset value) {
        _value = value;
      }
    
      Offset get value => _value;
    }
    
    @MappableClass()
    class WithParts with WithPartsMappable {
      late final String _id;
      late final Map<int, Part> _parts;
    
      WithParts(String id, Map<int, Part> parts) {
        _id = id;
        _parts = parts;
      }

      Map<int, Part> get parts => _parts;

      String get id {
        return _id.toLowerCase();
      }
    }

    void main() {
        PartMapper.ensureInitialized();
        WithPartsMapper.ensureInitialized();
        MapperContainer.globals.use(MapMapper<HashMap>(
          <K, V>(Map<K, V> m) => HashMap.of(m),
          <K, V>(f) => f<HashMap<K, V>>(),
        ));
        Map<int, Part> parts = Map<int, Part>.from({
          1: Part('part1', Offset(1, 0)),
          2: Part('part2', Offset(2, 0)),
        });

        var wp1 = WithParts('id1', parts);
        var wp1AsJson = wp1.toJson();
        var wp1FromJson = WithPartsMapper.fromJson(wp1AsJson);
        assert(wp1 == wp1FromJson);
        assert(!identical(wp1, wp1FromJson));
    }

Bonus question: do I need a custom mapper for Offset?