Moved away from singleton pattern

Signed-off-by: Hans Kokx <hans.d.kokx@gmail.com>

packages/wolf_3d_flutter/lib/wolf_3d.dart

@@ -6,9 +6,7 @@ import 'package:wolf_3d_dart/wolf_3d_engine.dart';
 import 'package:wolf_3d_dart/wolf_3d_synth.dart';
 
 class Wolf3d {
-  Wolf3d._();
-  static final Wolf3d _instance = Wolf3d._();
-  static Wolf3d get I => _instance;
+  Wolf3d();
 
   // --- State ---
   final List<WolfensteinData> availableGames = [];
@@ -31,6 +29,13 @@ class Wolf3d {
   int get activeEpisode => _activeEpisode;
 
   void setActiveEpisode(int episodeIndex) {
+    if (_activeGame == null) {
+      throw StateError("No active game selected. Call setActiveGame() first.");
+    }
+    if (episodeIndex < 0 || episodeIndex >= _activeGame!.episodes.length) {
+      throw RangeError("Episode index out of range for the active game.");
+    }
+
     _activeEpisode = episodeIndex;
   }
 
@@ -45,12 +50,22 @@ class Wolf3d {
 
   // --- Actions ---
   void setActiveGame(WolfensteinData game) {
+    if (!availableGames.contains(game)) {
+      throw ArgumentError(
+        "The provided game data is not in the list of available games.",
+      );
+    }
+
+    if (_activeGame == game) {
+      return; // No change needed
+    }
+
     _activeGame = game;
     audio.activeGame = game;
   }
 
   /// Initializes the engine by loading available game data.
-  Future<void> init({String? directory}) async {
+  Future<Wolf3d> init({String? directory}) async {
     await audio.init();
     availableGames.clear();
 
@@ -63,7 +78,6 @@ class Wolf3d {
     for (final version in versionsToTry) {
       try {
         final ext = version.version.fileExtension;
-        // final folder = 'assets/${version.path}';
         final folder = 'packages/wolf_3d_assets/assets/${version.path}';
 
         final data = WolfensteinLoader.loadFromBytes(
@@ -100,6 +114,8 @@ class Wolf3d {
         debugPrint("External discovery failed: $e");
       }
     }
+
+    return this;
   }
 
   Future<ByteData?> _tryLoad(String path) async {