type 'int' is not a subtype of type 'String Range slider with data from API

I am trying to make range slider where values are loaded before the app is completed and also i am trying to get value from the API, not hardcoded it, so I create such function which should loaded values before app is completed and trying to get values from API, but still facing with issues about cast to types:

 static Map? gainMin;

var _currentRangeValues = RangeValues(
      num.parse(gainMin?['min']).round().toDouble(),
      num.parse(gainMin?['max']).round().toDouble());

  @override
  void initState() {
    WidgetsBinding.instance.addPostFrameCallback((_) {
      fetchGains(num.parse(gainMin?['min']).round().toDouble(),
          num.parse(gainMin?['max']).round().toDouble());
    });

    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder<Map?>(
      future: fetchGains(_currentRangeValues.start.round().toInt(),
          _currentRangeValues.end.round().toInt()),
      builder: (context, snapshot) {
        print(gainMin);
RangeSlider(
            values: RangeValues(num.parse(gainMin?['min']).roundToDouble(),
                num.parse(gainMin?['max']).roundToDouble()),
            min: num.parse(gainMin?['min']).roundToDouble(),
            max: num.parse(gainMin?['max']).roundToDouble(),
            activeColor: Colors.green,
            inactiveColor: Colors.green,
            onChanged: (RangeValues value) {
              setState(() {
                if (_debounce?.isActive ?? false) _debounce?.cancel();
                _debounce = Timer(const Duration(milliseconds: 15), () {
                  snapshot.data?['value'] = value.start;
                  snapshot.data?['valueMax'] = value.end;
                  // _currentRangeValues = value;
                });
              });
            },
          )
}
}

But I got the error: type 'int' is not a subtype of type 'String' How can i solve this issue?

Print returns {min: 1, max: 22, value: 0.0, valueMax: 0.0, step: 1.0}