Flutter animated text as counter, double not accepted - flutter

Question

Based on this repo: https://gist.github.com/bluemix/4c932e8c4a1cd6f497a4353d9e536f57
Class:
class AnimatedCount extends ImplicitlyAnimatedWidget {
const AnimatedCount({
Key? key,
required this.count,
Duration duration = const Duration(milliseconds: 600),
Curve curve = Curves.fastOutSlowIn,
}) : super(duration: duration, curve: curve, key: key);
final num count;
#override
ImplicitlyAnimatedWidgetState<ImplicitlyAnimatedWidget> createState() {
return _AnimatedCountState();
}
}
class _AnimatedCountState extends AnimatedWidgetBaseState<AnimatedCount> {
IntTween _intCount = IntTween(begin: 0, end: 1);
Tween<double> _doubleCount = Tween<double>();
#override
Widget build(BuildContext context) {
return widget.count is int
? Text(_intCount.evaluate(animation).toString())
: Text(_doubleCount.evaluate(animation).toStringAsFixed(2));
}
#override
void forEachTween(TweenVisitor<dynamic> visitor) {
if (widget.count is int) {
_intCount = visitor(
_intCount,
widget.count,
(dynamic value) => IntTween(begin: value),
) as IntTween;
} else {
_doubleCount = visitor(
_doubleCount,
widget.count,
(dynamic value) => Tween<double>(begin: value),
) as Tween<double>;
}
}
}
I'm trying to pass data:
AnimatedCount(count: getKcal()), //as Text Widget
getKcal function is easy (as for testing purposes):
double getKcal() {
return 10;
}
However I receive an exception here (on the Class AnimatedCount):
_doubleCount = visitor(
Exception has occurred.
_CastError (type 'Null' is not a subtype of type 'double' in type cast)
No problems when I use integers. The Class does not accept doubles and I'm not finding why...