Tapping to place a bullet works fine. Although dragging on the screen breaks something which then breaks the placement of bullets even with tapping.

I temporarily fixed it by commenting out

  HorizontalDragGestureRecognizer horizontalDragGestureRecognizer =
      new HorizontalDragGestureRecognizer();

  Flame.util.addGestureRecognizer(horizontalDragGestureRecognizer
    ..onUpdate = (startDetails) => game.dragInput(startDetails.globalPosition));

in main.dart.

Although, out of curiosity - I'd wanna find out why it breaks?

I'm running it on a Pixel 1 via USB Debugging.

Performing hot restart...
Restarted application in 1,239ms.
[VERBOSE-2:ui_dart_state.cc(157)] Unhandled Exception: ServicesBinding.defaultBinaryMessenger was accessed before the binding was initialized.
If you're running an application and need to access the binary messenger before runApp() has been called (for example, during plugin initialization), then you need to explicitly call the WidgetsFlutterBinding.ensureInitialized() first.
If you're running a test, you can call the TestWidgetsFlutterBinding.ensureInitialized() as the first line in your test's main() method to initialize the binding.

flutter: ══╡ EXCEPTION CAUGHT BY SCHEDULER LIBRARY ╞═════════════════════════════════════════════════════════
flutter: The following ConcurrentModificationError was thrown during a scheduler callback:
flutter: Concurrent modification during iteration: Instance(length:29) of '_GrowableList'.
flutter:
flutter: When the exception was thrown, this was the stack:
flutter: #0 List.forEach (dart:core-patch/growable_array.dart:286:36)
flutter: #1 Bullet.update (package:galaxygame/bullet.dart:21:18)
flutter: #2 BaseGame.update. (package:flame/game.dart:228:33)
flutter: #3 IterableMixin.forEach (dart:collection/iterable.dart:46:30)
flutter: #4 BaseGame.update (package:flame/game.dart:228:16)
flutter: #5 Galaxy.update (package:galaxygame/galaxy.dart:50:11)
flutter: #6 _GameRenderBox._update (package:flame/game.dart:124:10)
flutter: #7 _GameRenderBox._tick (package:flame/game.dart:117:5)
flutter: #8 SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:1102:15)
flutter: #9 SchedulerBinding.handleBeginFrame. (package:flutter/src/scheduler/binding.dart:1017:11)
flutter: #10 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:379:8)
flutter: #11 SchedulerBinding.handleBeginFrame (package:flutter/src/scheduler/binding.dart:1015:17)
flutter: #12 SchedulerBinding._handleBeginFrame (package:flutter/src/scheduler/binding.dart:949:5)
flutter: #16 _invoke1 (dart:ui/hooks.dart:273:10)
flutter: #17 _beginFrame (dart:ui/hooks.dart:200:3)
flutter: (elided 3 frames from package dart:async)