Remove NoopInAppPurchase implementation

packages/in_app_purchase/in_app_purchase_platform_interface/lib/src/in_app_purchase_platform.dart

@@ -6,7 +6,6 @@ import 'dart:async';
 
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 
-import 'noop_in_app_purchase.dart';
 import 'types/types.dart';
 
 /// The interface that implementations of in_app_purchase must implement.
@@ -22,10 +21,18 @@ abstract class InAppPurchasePlatform extends PlatformInterface {
 
   static final Object _token = Object();
 
-  static InAppPurchasePlatform _instance = NoopInAppPurchase();
+  static InAppPurchasePlatform? _instance;
 
   /// The default instance of [InAppPurchasePlatform] to use.
-  static InAppPurchasePlatform get instance => _instance;
+  static InAppPurchasePlatform get instance {
+    final InAppPurchasePlatform? platform = _instance;
+    if (platform == null) {
+      throw UnimplementedError(
+          'No platform specific implementation set. Please make sure you set the `instance` with a valid platform specific implementation of the `InAppPurchasePlatform` class.');
+    }
+
+    return platform;
+  }
 
   /// Platform-specific plugins should set this with their own platform-specific
   /// class that extends [InAppPurchasePlatform] when they register themselves.

packages/in_app_purchase/in_app_purchase_platform_interface/test/in_app_purchase_platform_test.dart

@@ -4,16 +4,15 @@
 
 import 'package:flutter_test/flutter_test.dart';
 import 'package:in_app_purchase_platform_interface/in_app_purchase_platform_interface.dart';
-import 'package:in_app_purchase_platform_interface/src/noop_in_app_purchase.dart';
 import 'package:mockito/mockito.dart';
 import 'package:plugin_platform_interface/plugin_platform_interface.dart';
 
 void main() {
   TestWidgetsFlutterBinding.ensureInitialized();
 
   group('$InAppPurchasePlatform', () {
-    test('$NoopInAppPurchase is the default instance', () {
-      expect(InAppPurchasePlatform.instance, isA<NoopInAppPurchase>());
+    test('default instance is null and throws unimplemented exception', () {
+      expect(() => InAppPurchasePlatform.instance, throwsUnimplementedError);
     });
 
     test('Cannot be implemented with `implements`', () {