CallBacks error : The method 'call' was called on null - flutter

Question

I am currently working on this tutorial in which transfers a variable to an external file to another (with a callback)
but i have this error :
======== Exception caught by gesture ===============================================================
The following NoSuchMethodError was thrown while handling a gesture:
The method 'call' was called on null.
Receiver: null
Tried calling: call(3)
When the exception was thrown, this was the stack:
#0 Object.noSuchMethod (dart:core-patch/object_patch.dart:54:5)
#1 Son.build.<anonymous closure> (package:montest/son.dart:24:21)
#2 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
#3 TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:607:11)
#4 BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:296:5)
...
Handler: "onTap"
Recognizer: TapGestureRecognizer#cf4d9
debugOwner: GestureDetector
state: possible
won arena
finalPosition: Offset(178.3, 84.9)
button: 1
sent tap down
====================================================================================================
son.dart
///
/// Inside the Child Widget
///
import 'package:flutter/material.dart';
// Step 1: Define a Callback.
typedef void IntCallback(int id);
class Son extends StatelessWidget {
// Step 2: Configre the child to expect a callback in the constructor(next 2 lines):
final IntCallback onSonChanged;
Son({ #required this.onSonChanged });
int elementId = 3;
#override
Widget build(BuildContext context) {
return ElevatedButton(
onPressed: () {
// Step 3: On specific action(e.g onPressed/
// onTap/onLoad.. onWhatEver) trigger the callback
// with the data you want to pass to the parent.
// Data will be passed as parameter(see elementId):
onSonChanged(elementId);
// Done in the child.
},
child: Text('Click me to call the callback!'),
);
}
}
///
///////////////
parent.dart
import 'son.dart';
///
/// Inside the Parent Widget
///
import 'package:flutter/material.dart';
class Father extends StatefulWidget {
#override
_FatherState createState() => _FatherState();
}
class _FatherState extends State<Father> {
// Step 1 (optional): Define a Global variable
// to store the data comming back from the child.
int id;
// Step 2: Define a function with the same signature
// as the callback, so the callback will point to it,
// this new function will get the data from the child,
// set it to the global variable (from step 1)
// in the parent, and then update the UI by setState((){});
void updateId(int newId) {
setState(() {
id = newId;
});
}
#override
Widget build(BuildContext context) {
return Container(
// Step 3: Construct a child widget and pass the
child: Son(
// Many options to make onSonChanged points to
// an executable code(function) within memory
// called 'updateId':
//
// 1st option:
onSonChanged: (int newId) {
updateId(newId);
},
// 2nd option: onSonChanged: updateId,
// 3rd option: onSonChanged: (int newId) => updateId(newId)
// So each time the 'onSonChanged' called by the action
// we defined inside the child, a new data will be
// passed to this parent.
)
);
}
}
Thanks for your help

Answer 1

make id nullable and check it.
try this
import 'package:flutter/material.dart';
typedef void IntCallback(int id);
class Son extends StatelessWidget {
final IntCallback onSonChanged;
Son({required this.onSonChanged});
int elementId = 3;
#override
Widget build(BuildContext context) {
return ElevatedButton(
onPressed: () {
onSonChanged(elementId);
},
child: Text('Click me to call the callback!'),
);
}
}
class Father extends StatefulWidget {
#override
_FatherState createState() => _FatherState();
}
class _FatherState extends State<Father> {
int? id;
void updateId(int newId) {
setState(() {
id = newId;
});
}
#override
Widget build(BuildContext context) {
return Column(
children: [
id == null ? Text(" son value: null") : Text("son value: $id"),
Container(
child: Son(
onSonChanged: (int newId) {
updateId(newId);
},
),
),
],
);
}
}