Why this "Null check operator used on a null value error" coming in Flutter

I'm using GetX for bottom sheet purpose. i used this below code to implement the functionality. when i press the (i)-info icon i want the bottom sheet to pop-up but instead i'm getting this error.

The following _CastError was thrown while handling a gesture: Null check operator used on a null value

This is the code:

GestureDetector(
    onTap:(){
      Get.bottomSheet(
        Container(
          child: Wrap(
              children:<Widget>[
                ListTile(
                  leading: Icon(Icons.wb_incandescent),
                  title: Text("Testing"),
                  onTap: () => {Get.changeTheme(ThemeData.light())},
                ),
                ListTile(
                  leading: Icon(Icons.wb_incandescent),
                  title: Text("Testing"),
                  onTap: () => {Get.changeTheme(ThemeData.dark())},
                ),
              ]
          ),
        ),
      );
    },
    child: Container(
      width: 30.0,
      height: 30.0,
      decoration: BoxDecoration(
        color: Color(0xFF1D265F),
        borderRadius: BorderRadius.circular(80.0),
      ),

        child: new LayoutBuilder(
          builder: (context, constraint) {
            return new Icon(
              Icons.info_outline_rounded,
              color: widget.cardContent.cardColor,
              size: constraint.biggest.height,
            );
          },
        ),
    ),
  ),