overlayColor property Null safety
The highlight color that's typically used to indicate that the slider thumb is focused, hovered, or dragged.
If this property is null, Slider will use activeColor with with an opacity of 0.12, If null, SliderThemeData.overlayColor will be used.
If that is also null, If ThemeData.useMaterial3 is true, Slider will use ColorScheme.primary with an opacity of 0.08 when slider thumb is hovered and with an opacity of 0.12 when slider thumb is focused or dragged, If ThemeData.useMaterial3 is false, defaults to ColorScheme.primary with an opacity of 0.12.
final MaterialStateProperty<Color?>? overlayColor;