Flare Actor suddenly disappears when using controller parameter flutter

I tried using flare in my application to design an rating screen with a progress bar in it and I am encountering an error "whenever I try to add controller parameter the flare suddenly disappears" with an error.

The getter 'duration' was called on null.

Receiver: null

Tried calling: duration

I have attached the required code below

  FlareRateController _flareController;

void updateDragPosition(Offset offset) {
    setState(() {
      _dragPercent = (offset.dx / sliderWidth).clamp(0.0, 1.0);
      _flareController.updatePercent(_dragPercent);
    });
  }

void initState() {
    super.initState();
    _flareController = FlareRateController();
    SystemChrome.setEnabledSystemUIOverlays([]);
  }

_buildFlareActor() => SizedBox(
        width: 1100.w,
        height: 680.h,
        child: FlareActor(
          "assets/rate.flr",
          artboard: "Artboard",
          controller: _flareController,
        ),
      );

The FlareRatecontroller class is

class FlareRateController extends FlareController {
  FlutterActorArtboard _artboard;
  ActorAnimation _rateAnimation;

  double _slidePercent = 0.0;
  double _currentSlide = 0.0;
  double _smoothTime = 5;

  void updatePercent(double val) {
    _slidePercent = val;
  }

  @override
  void initialize(FlutterActorArtboard artboard) {
    if (artboard.name.compareTo("Artboard") == 0) {
      _artboard = artboard;
      _rateAnimation = artboard.getAnimation("Slide");
    }
  }

  @override
  bool advance(FlutterActorArtboard artboard, double elapsed) {
    if (artboard.name.compareTo("Artboard") == 0) {
      _currentSlide += (_slidePercent - _currentSlide) *
          math.min(
            1,
            elapsed * _smoothTime,
          );
      _rateAnimation.apply(
          _currentSlide * _rateAnimation.duration, artboard, 1); //error here in duration
    }
    return true;
  }

  @override
  void setViewTransform(Mat2D viewTransform) {
    // TODO: implement setViewTransform
  }
}

the error is in _rateAnimation.duration (already commented the lie)

Your Flare Actor disappears because the propertie "duration" return null

I found this rate app speed code and copy, because i need more information about the code. That's what i found:

Your code is getting animation for your ActorAnimation like "Slide". Try changing the "Slide" parameter and set "slide" (s in lower case)

Then run your app again and tell me if u got it.