fix cast NPE in invokeListMethod and invokeMapMathod (#30760)

packages/flutter/lib/src/services/platform_channel.dart

@@ -314,7 +314,7 @@ class MethodChannel {
   ///  * [invokeMethod], which this call delegates to.
   Future<List<T>> invokeListMethod<T>(String method, [ dynamic arguments ]) async {
     final List<dynamic> result = await invokeMethod<List<dynamic>>(method, arguments);
-    return result.cast<T>();
+    return result?.cast<T>();
   }
 
   /// An implementation of [invokeMethod] that can return typed maps.
@@ -328,7 +328,7 @@ class MethodChannel {
   ///  * [invokeMethod], which this call delegates to.
   Future<Map<K, V>> invokeMapMethod<K, V>(String method, [ dynamic arguments ]) async {
     final Map<dynamic, dynamic> result = await invokeMethod<Map<dynamic, dynamic>>(method, arguments);
-    return result.cast<K, V>();
+    return result?.cast<K, V>();
   }
 
   /// Sets a callback for receiving method calls on this channel.

packages/flutter/test/services/platform_channel_test.dart

@@ -69,6 +69,21 @@ void main() {
       expect(await channel.invokeListMethod<String>('sayHello', 'hello'), <String>['hello', 'world']);
     });
 
+    test('can invoke list method and get null result', () async {
+      BinaryMessages.setMockMessageHandler(
+        'ch7',
+        (ByteData message) async {
+          final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message);
+          if (methodCall['method'] == 'sayHello') {
+            return jsonMessage.encodeMessage(<dynamic>[null]);
+          } else {
+            return jsonMessage.encodeMessage(<dynamic>['unknown', null, null]);
+          }
+        },
+      );
+      expect(await channel.invokeListMethod<String>('sayHello', 'hello'), null);
+    });
+
 
     test('can invoke map method and get result', () async {
       BinaryMessages.setMockMessageHandler(
@@ -86,6 +101,21 @@ void main() {
       expect(await channel.invokeMapMethod<String, String>('sayHello', 'hello'), <String, String>{'hello': 'world'});
     });
 
+    test('can invoke map method and get null result', () async {
+      BinaryMessages.setMockMessageHandler(
+        'ch7',
+        (ByteData message) async {
+          final Map<dynamic, dynamic> methodCall = jsonMessage.decodeMessage(message);
+          if (methodCall['method'] == 'sayHello') {
+            return jsonMessage.encodeMessage(<dynamic>[null]);
+          } else {
+            return jsonMessage.encodeMessage(<dynamic>['unknown', null, null]);
+          }
+        },
+      );
+      expect(await channel.invokeMapMethod<String, String>('sayHello', 'hello'), null);
+    });
+
     test('can invoke method and get error', () async {
       BinaryMessages.setMockMessageHandler(
         'ch7',