Exclamation mark in front of await

Question

One case I've looked up where this is used is Navigator with WillPopScope as the parent as mentioned in the comment of this answer.

To put my question concretely, the following code should pop from the nested navigator but instead the app exits when the back button is pressed on android.

void main() {
  runApp(MaterialApp(title: 'Navigation Basics', home: PearlScreen()));
}


class PearlScreen extends StatefulWidget {
  const PearlScreen({Key? key}) : super(key: key);

  @override
  State<PearlScreen> createState() => _PearlScreenState();
}

class _PearlScreenState extends State<PearlScreen> {
  final _navigatorKey = GlobalKey<NavigatorState>();

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: () async => await _navigatorKey.currentState!.maybePop(),
      child: Navigator(
        key: _navigatorKey,
        onGenerateRoute: (settings) {
          Widget screen;
          switch (settings.name) {
            case '/':
              screen = const HomeScreen();
              break;
            case '/quiz':
              screen = const QuizScreen();
              break;
            default:
              throw "Invalid route in Pearl: ${settings.name}";
          }
          return MaterialPageRoute(
            builder: (_) => Material(child: screen),
          );
        },
      ),
    );
  }
}

But as soon as I change it to onWillPop: () async => !await _navigatorKey.currentState!.maybePop(), it works correctly and pops from the nested navigator instead of exiting the app.

What is the significance of !await?