rangeTrackShape property Null safety
The shape that will be used to draw the RangeSlider's track.
The SliderTrackShape.getPreferredRect method is used to map slider-relative gesture coordinates to the correct thumb position on the track. It is also used to horizontally position the tick marks, when the slider is discrete.
The default value is RoundedRectRangeSliderTrackShape.
final RangeSliderTrackShape? rangeTrackShape;