flutter_japFlameTutorial_flyAdd.dart

...

class FlyTerminator extends Game {
  ...
  List<Fly>? flies;
  Random? rnd;

  FlyTerminator() {
    initialize();
  }

  void spawnFly() {
    double x = rnd!.nextDouble() * (screenSize!.width - tileSize!);
    double y = rnd!.nextDouble() * (screenSize!.height - tileSize!);
    flies!.add(Fly(this, x, y));
  }

  void initialize() async {
    // flies = List<Fly>(); // returns null
    flies = List<Fly>.empty(growable: true); // returns [ ]
    ...
    rnd = Random();
    spawnFly();
  }

  void render(Canvas canvas) {
    ...
    flies!.forEach((Fly fly) => fly.render(canvas));
  }

  void update(double t) {
    flies!.forEach((Fly fly) => fly.update(t));
    ...
  }
  ...
}