ERROR: "Exception caught by gesture" in Flutter - flutter

Question

I am trying to store data in a class to store this information and use them later, but I am getting this error message and can't find out what I have to change to make it work. Here is a simplified version of my code where I get the same error.
class LUI {
String lui;
LUI({
this.lui,
});
}
class Test extends StatefulWidget {
final LUI data;
Test({
Key key,
this.data,
});
#override
_TestState createState() => _TestState();
}
class _TestState extends State<Test> {
TextEditingController _controller = new TextEditingController();
#override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: [
Container(
child: Container(
child: TextField(
controller: _controller,
keyboardType: TextInputType.number,
))),
TextButton(
child: Text('add information'),
onPressed: () {
_sendresult(context);
print(widget.data.lui);
},
),
],
),
),
);
}
void _sendresult(BuildContext context) {
final result = _controller.text;
print(result);
setState(() {
widget.data.lui = result;
});
}
}```

Answer 1

The problem is that, the Test class expects a value to be passed for the data argument. However as you are not passing any value to that while creating the instance, which makes the field to remain null.
Later when you access the same field in the print(widget.data.lui); it results in a null pointer exception. As you already ahve figured, the soultion is to pass a value to data.
However, you can write your code in a better way so that you can catch this error in the compile time rather than waiting till the runtime.
Approach 1: (With Null-Safety enabled)
Use the keyword required to specify the filed is a must e.g.
Test({
Key key,
required this.data,
});
Now, with this, creating an instance like Test() with arguments input will throw compilation errors compalining about the missing input argument.
Approach 2: (Without Null-Safety enabled)
Use the annotation #required to specify the filed is a required e.g.
Test({
Key key,
#required this.data,
});
In this case, failing to pass an argument will throw a compile time warning (yellow line highlighter) which unlike the above case will inform you but will not block your execution.

Answer 2

I had to send a value for the data parameter from where I create the instance of the Test class. Thanks, Sisir for your answer.
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Basis',
theme: ThemeData(
primaryColor: Colors.red,
textTheme: TextTheme(
headline1: TextStyle(
fontSize: 40,
fontWeight: FontWeight.w600,
color: Colors.black,
),
),
),
home: Test(data: new LUI()));
}
}