Async/await instead of then

Do

void getActiveUserCount() async {
  try {
    var users = await getActiveUsers();
    debugPrint(users.length.toString());
  }
  catch (error)
  {
    debugPrint(error.toString());
  }
}

Don't

void getActiveUserCount() {
  getActiveUsers().then((user) {
    debugPrint(users.length.toString());
  }).catchError((error) {
    debugPrint(error.toString());
  }
}

Conclusion: There's no meaningful difference between the two versions of your code. Both achieve the same result. But await can often save you a bunch of lines of code making your code simpler to read, test and maintain.