Fixed bug where we could accidently call a callback twice. (#43467)

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

@@ -200,6 +200,7 @@ class _DefaultBinaryMessenger extends BinaryMessenger {
         response = await handler(data);
       } else {
         ui.channelBuffers.push(channel, data, callback);
+        callback = null;
       }
     } catch (exception, stack) {
       FlutterError.reportError(FlutterErrorDetails(
@@ -209,9 +210,11 @@ class _DefaultBinaryMessenger extends BinaryMessenger {
         context: ErrorDescription('during a platform message callback'),
       ));
     } finally {
+      if (callback != null) {
         callback(response);
       }
     }
+  }
 
   @override
   Future<ByteData> send(String channel, ByteData message) {

packages/flutter/test/services/default_binary_messenger_test.dart

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:convert';
+import 'dart:typed_data';
+import 'dart:ui' as ui;
+import 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  TestWidgetsFlutterBinding.ensureInitialized();
+
+  ByteData _makeByteData(String str) {
+    final List<int> list = utf8.encode(str);
+    final ByteBuffer buffer =
+        list is Uint8List ? list.buffer : Uint8List.fromList(list).buffer;
+    return ByteData.view(buffer);
+  }
+
+  test('default binary messenger calls callback once', () async {
+    int count = 0;
+    const String channel = 'foo';
+    ServicesBinding.instance.defaultBinaryMessenger.handlePlatformMessage(
+        channel, _makeByteData('bar'), (ByteData message) async {
+      count += 1;
+      return null;
+    });
+    expect(count, equals(0));
+    await ui.channelBuffers.drain(channel,
+        (ByteData data, ui.PlatformMessageResponseCallback callback) {
+      callback(null);
+      return null;
+    });
+    expect(count, equals(1));
+  });
+}

packages/flutter_web_plugins/lib/src/plugin_registry.dart

@@ -82,6 +82,7 @@ class _PlatformBinaryMessenger extends BinaryMessenger {
         response = await handler(data);
       } else {
         ui.channelBuffers.push(channel, data, callback);
+        callback = null;
       }
     } catch (exception, stack) {
       FlutterError.reportError(FlutterErrorDetails(
@@ -91,9 +92,11 @@ class _PlatformBinaryMessenger extends BinaryMessenger {
         context: ErrorDescription('during a plugin platform message call'),
       ));
     } finally {
+      if (callback != null) {
         callback(response);
       }
     }
+  }
 
   /// Sends a platform message from the platform side back to the framework.
   @override