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You can also call the completeError method of a Completer when you need to deal with an error:

1. Change the code in the calculate() method like this:

calculate() async {
    try {
      await new Future.delayed(const Duration(seconds : 5));
      completer.complete(42);
    }
    catch (_) {
      completer.completeError(null);
    }
  }

2. In the call to getNumber, you could then concatenate a catchError to the then function:

getNumber().then((value) {
                setState(() {
                  result = value.toString();
                });
              }).catchError((e) {
                result = 'An error occurred';
              });