fillColor property Null safety
final
The fill color for selected toggle buttons.
If this property is null, then ToggleButtonTheme.of(context).fillColor is used. If ToggleButtonsThemeData.fillColor is also null, then the fill color is null.
If fillColor is a MaterialStateProperty<Color>, then MaterialStateProperty.resolve is used for the following MaterialStates:
Implementation
final Color? fillColor;