fillColor property Null safety

MaterialStateProperty<Color?>? fillColor
final

The color that fills the checkbox, in all MaterialStates.

Resolves in the following states:

link
This example resolves the fillColor based on the current MaterialState of the Checkbox, providing a different Color when it is MaterialState.disabled. 
Checkbox(
  value: true,
  onChanged: (_){},
  fillColor: MaterialStateProperty.resolveWith<Color>((Set<MaterialState> states) {
    if (states.contains(MaterialState.disabled)) {
      return Colors.orange.withOpacity(.32);
    }
    return Colors.orange;
  })
)

If null, then the value of activeColor is used in the selected state. If that is also null, the value of CheckboxThemeData.fillColor is used. If that is also null, then ThemeData.disabledColor is used in the disabled state, ThemeData.toggleableActiveColor is used in the selected state, and ThemeData.unselectedWidgetColor is used in the default state.

Implementation

final MaterialStateProperty<Color?>? fillColor;