ListTile key not found in Flutter - flutter

Question

I have a List defined as below and I'm trying to highlight one element of the list during the tutorial. I am currently using Tutorial Coach Mark. Every time I debug my app it shows me
KEY information could not be obtained
I don't know why this is happening as I've followed all the necessary steps for integration shown in the dependency.
This is my code:
class _SettingsState extends State<Settings> {
TutorialCoachMark tutorialCoachMark;
GlobalKey keyButton3 = GlobalKey();
List<TargetFocus> targets = List();
void initState() {
initTarget();
WidgetsBinding.instance.addPostFrameCallback(_afterLayouts);
super.initState();
}
void _afterLayouts(_) {
Future.delayed(Duration(milliseconds: 100), () {
showTutorial(context);
});
}
void showTutorial(context) {
tutorialCoachMark = TutorialCoachMark(context,
targets: targets,
colorShadow: Colors.red,
textSkip: "SKIP",
paddingFocus: 10,
opacityShadow: 0.8, onFinish: () {
print("finish");
}, onClickTarget: (target) {
print(target);
}, onClickSkip: () {
print("skip");
})
..show();
}
void initTarget() {
targets.add(
TargetFocus(
//claim reward
identify: "Target 2",
keyTarget: keyButton3,
color: Colors.purple,
contents: [
ContentTarget(
align: AlignContent.bottom,
child: Container(
child: Column(
mainAxisSize: MainAxisSize.min,
crossAxisAlignment: CrossAxisAlignment.start,
children: <Widget>[
Text(
"Click here",
style: TextStyle(
fontWeight: FontWeight.bold,
color: Colors.white,
fontSize: 20.0),
),
],
),
))
],
shape: ShapeLightFocus.RRect,
radius: 6),
);
}
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
centerTitle: true,
iconTheme: IconThemeData(
color: Colors.white,
),
elevation: 0,
title: Text(
'Settings',
style: kTasksStyle,
),
backgroundColor: Color(0xff0C7368),
),
body: Stack(
children: [
Padding(
padding: const EdgeInsets.only(top: 8),
child: ListView.separated(
separatorBuilder: (context, index) => Divider(
height: 2,
color: Colors.grey.withOpacity(0.3),
),
itemCount: 5,
itemBuilder: (context, index) {
return Padding(
padding: EdgeInsets.all(5.0),
child: ListTile(
key: index == 0 ? keyButton3 : null,// to highlight the 0th index of the list only
title: Text(
"Hello",
style: kTasksStyle.copyWith(
fontSize: 15,
color: Colors.black,
fontWeight: FontWeight.normal),
),
leading: Icon(
Icons.supervised_user_circle,
color: Color(0xff0C7368),
),
),
);
}),
)
],
));
}
}
I am trying to achieve something similar to this: