Throw on invalid platform return type (#62)

## Problem (PFL-69)
If the type returned from the platform side is not the expected one, `invokeMethod returns null`. We are handling it by returning default values.

## Changes
As this should never happen, and if it was to, it would be a bug, throw an exception instead of hiding the issue.

lib/src/player.dart

@@ -123,15 +123,21 @@ class Player with PlayerEventHandler implements PlayerApi {
 
   // Can be used to call methods on the platform side that return a single
   // primitive value that is natively supported by the method channel.
-  Future<T?> _invokeMethod<T>(
+  Future<T> _invokeMethod<T>(
     String methodName, [
     dynamic data,
   ]) async {
-    final result = await _initializationResult;
-    if (!result) {
-      return Future.error('Error initializing player on native platform side.');
+    final initSuccess = await _initializationResult;
+    if (!initSuccess) {
+      throw Exception('Error initializing player on native platform side.');
     }
-    return _methodChannel.invokeMethod<T>(methodName, _buildPayload(data));
+    final payload = _buildPayload(data);
+    final result = await _methodChannel.invokeMethod<T>(methodName, payload);
+    if (result is! T) {
+      // result is T?, if it `is` not T => T is not nullable and result is null.
+      throw Exception('Native $methodName returned null.');
+    }
+    return result;
   }
 
   // Can be used to call methods on the platform side that return a complex
@@ -143,7 +149,7 @@ class Player with PlayerEventHandler implements PlayerApi {
   ]) async {
     final result = await _initializationResult;
     if (!result) {
-      return Future.error('Error initializing player on native platform side.');
+      throw Exception('Error initializing player on native platform side.');
     }
 
     final jsonString = await _methodChannel.invokeMethod<String>(
@@ -228,31 +234,28 @@ class Player with PlayerEventHandler implements PlayerApi {
 
   @override
   Future<double> get currentTime async =>
-      await _invokeMethod<double>(Methods.currentTime) ?? 0.0;
+      _invokeMethod<double>(Methods.currentTime);
 
   @override
-  Future<double> get duration async =>
-      await _invokeMethod<double>(Methods.duration) ?? 0.0;
+  Future<double> get duration async => _invokeMethod<double>(Methods.duration);
 
   @override
   Future<double> get timeShift async =>
-      await _invokeMethod<double>(Methods.getTimeShift) ?? 0.0;
+      _invokeMethod<double>(Methods.getTimeShift);
 
   @override
   Future<void> setTimeShift(double timeShift) async =>
       _invokeMethod<void>(Methods.setTimeShift, timeShift);
 
   @override
   Future<double> get maxTimeShift async =>
-      await _invokeMethod<double>(Methods.maxTimeShift) ?? 0.0;
+      _invokeMethod<double>(Methods.maxTimeShift);
 
   @override
-  Future<bool> get isLive async =>
-      await _invokeMethod<bool>(Methods.isLive) ?? false;
+  Future<bool> get isLive async => _invokeMethod<bool>(Methods.isLive);
 
   @override
-  Future<bool> get isPlaying async =>
-      await _invokeMethod<bool>(Methods.isPlaying) ?? false;
+  Future<bool> get isPlaying async => _invokeMethod<bool>(Methods.isPlaying);
 
   @override
   Future<List<SubtitleTrack>> get availableSubtitles async =>