rangeTickMarkShape property Null safety
The shape that will be used to draw the RangeSlider's tick marks.
The RangeSliderTickMarkShape.getPreferredSize is used to help determine the location of each tick mark on the track. The slider's minimum size will be at least this big.
The default value is RoundRangeSliderTickMarkShape.
final RangeSliderTickMarkShape? rangeTickMarkShape;