Prevent a potential infinite loop. (#66073)

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

@@ -309,13 +309,14 @@ class _DefaultBinaryMessenger extends BinaryMessenger {
 
   @override
   void setMessageHandler(String channel, MessageHandler? handler) {
-    if (handler == null)
+    if (handler == null) {
       _handlers.remove(channel);
-    else
+    } else {
       _handlers[channel] = handler;
-    ui.channelBuffers.drain(channel, (ByteData? data, ui.PlatformMessageResponseCallback callback) async {
-      await handlePlatformMessage(channel, data, callback);
-    });
+      ui.channelBuffers.drain(channel, (ByteData? data, ui.PlatformMessageResponseCallback callback) async {
+        await handlePlatformMessage(channel, data, callback);
+      });
+    }
   }
 
   @override

packages/flutter/test/services/platform_channel_test.dart

@@ -25,6 +25,7 @@ void main() {
       final String reply = await channel.send('hello');
       expect(reply, equals('hello world'));
     });
+
     test('can receive string message and send reply', () async {
       channel.setMessageHandler((String message) async => message + ' world');
       String reply;
@@ -170,13 +171,25 @@ void main() {
       expect(result, isNull);
     });
 
-    test('can handle method call with no registered plugin', () async {
+    test('can handle method call with no registered plugin (setting before)', () async {
       channel.setMethodCallHandler(null);
       final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
       ByteData envelope;
       await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
         envelope = result;
       });
+      await null; // just in case there's something async happening
+      expect(envelope, isNull);
+    });
+
+    test('can handle method call with no registered plugin (setting after)', () async {
+      final ByteData call = jsonMethod.encodeMethodCall(const MethodCall('sayHello', 'hello'));
+      ByteData envelope;
+      await ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage('ch7', call, (ByteData result) {
+        envelope = result;
+      });
+      channel.setMethodCallHandler(null);
+      await null; // just in case there's something async happening
       expect(envelope, isNull);
     });
 
@@ -262,6 +275,7 @@ void main() {
       expect(channel.checkMockMethodCallHandler(handler), true);
     });
   });
+
   group('EventChannel', () {
     const MessageCodec<dynamic> jsonMessage = JSONMessageCodec();
     const MethodCodec jsonMethod = JSONMethodCodec();
@@ -298,6 +312,7 @@ void main() {
       await Future<void>.delayed(Duration.zero);
       expect(canceled, isTrue);
     });
+
     test('can receive error event', () async {
       ServicesBinding.instance.defaultBinaryMessenger.setMockMessageHandler(
         'ch',