button add null color

lib/common_widgets/aku_material_button.dart

@@ -12,6 +12,7 @@ class AkuMaterialButton extends MaterialButton {
   final double radius;
   final EdgeInsets padding;
   final Color color;
+  final Color nullColor;
 
   final Widget child;
   AkuMaterialButton({
@@ -28,7 +29,8 @@ class AkuMaterialButton extends MaterialButton {
     this.padding = EdgeInsets.zero,
     @required this.child,
     this.color = Colors.transparent,
-  }) : super(
+  })  : nullColor = color.withOpacity(0.5),
+        super(
           key: key,
           onPressed: onPressed,
           elevation: elevation,