Callback function issue in flutter

Question

I have two callback functions in a stateful widget which are called by two different screens. Here is the how I declared them in the stateful widget(onChange and finalBetList);

class AppListView extends StatefulWidget {
  final ValueChanged onChange;
  final List<MatchList> matchList;
  final ValueChanged finalBetList;

  AppListView({this.onChange, this.matchList, this.finalBetList});

And this is how I implemented them in the state class of the widget. Even if I try to pass the same parameter onChange working perfectly but finalBetList getting this error. The method 'call' was called on null.Receiver: null Tried calling: call(1)

widget.onChange(counter);
widget.finalBetList(counter);

Everything is same, one of them is workiing but other one get the error. What I'm missing?

Edit

This is the code where I call my stateful widget from HomeScreen(which works perfect)

          Stack(children: [
              AppListView(
                matchList: matchList,
                //callback function brings the counter value from ListView class
                onChange: (value) {
                  setState(() {
                    counter = value;
                  });
                },
              ),

This is the code where I call my stateful widget from second secreen(which gets error)

body: Container(
        child: AppListView(
            onChange: (value) {},
            finalBetList: (value) {
              counter = value;
              setState(() {
                //counter = value;
              });
            },
            matchList: matchList),
      ),

Answer 1

TL;DR

Assign an empty body implemention to the finalBetList property

Stack(children: [
              AppListView(
                matchList: matchList,
                finalBetList: (value) {}, // <- NEW CODE IS HERE
                //callback function brings the counter value from ListView class
                onChange: (value) {
                  setState(() {
                    counter = value;
                  });
                },
              ),

---

Why should you do it?

In Dart, functions are objects. Objects, if not assigned a value, are null. In the code example from HomeScreen you assign a value to onChange, so later when you call it through widget.onChange(counter), onChange is evaluated as a function object and its body is executed.

However, in the same example you don't assign any value to finalBetList so it's left as null. When you make the call widget.finalBetList(counter), finalBetList is evaluated and it's clearly not a function object. At this point you'd except an error, but it's not the end of the story - as not only function objects can be called. Any class that implements the call() method can be called, too. So a call to finalBetList.call() is attempted. This is the exception you should get:

NoSuchMethodError: The method 'call' was called on null.
Receiver: null

Obviously null doesn't implement a call() method so it doesn't exists and an exception is thrown.

If you don't want to assign an empty body implemention to the finalBetList property inside HomeScreen, you also have the option to assign it in the constructor.

Another option I can think of is calling the function using conditional member access:

widget.finalBetList?.call(counter);

All function objects have the call() method which executes the function's body.