Dealing with errors using the then() callback:

The most obvious way to catch errors in a then() callback is using the catchError callback. To do so, follow these steps:

1. Add the following method to the the _FuturePageState class in the main.dart file:

  Future returnError() {
    throw ('Something terrible happened!');
  }

2. Whenever a method calls returnError, an error will be thrown. To catch this error, place the following code in the onPressed method of the ElevatedButton:

          returnError()
              .then((value){
                setState(() {
                  result = 'Success';
                });
              }).catchError((onError){
                setState(() {
                  result = onError;
                });
              }).whenComplete(() => print('Complete'));

3. Run the app and click the GO! button. You will see that the catchError callback was executed, updating the result State variable as shown in the following screenshot...