Fixes emitting new state on closed AssetsManager

lib/assets_manager/cubit/assets_manager_cubit.dart

@@ -19,6 +19,7 @@ class AssetsManagerCubit extends Cubit<AssetsManagerState> {
     /// do its job without adding too much delay for the user, we are letting
     /// the UI paint first, and then we start loading the assets.
     await Future<void>.delayed(const Duration(seconds: 1));
+    if (isClosed) return;
     final loadables = <Future<void> Function()>[
       _game.preFetchLeaderboard,
       ..._game.preLoadAssets(),

test/game/behaviors/ball_spawning_behavior_test.dart

@@ -131,6 +131,7 @@ void main() {
       flameTester.testGameWidget(
         'onNewState adds a ball',
         setUp: (game, _) async {
+          await game.onLoad();
           final behavior = BallSpawningBehavior();
           await game.pump([
             behavior,