Improve error message for uninitialized channel (#92758)

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

@@ -129,7 +129,17 @@ class MethodChannel {
   /// The messenger used by this channel to send platform messages.
   ///
   /// The messenger may not be null.
-  BinaryMessenger get binaryMessenger => _binaryMessenger ?? ServicesBinding.instance!.defaultBinaryMessenger;
+  BinaryMessenger get binaryMessenger {
+    assert(
+      _binaryMessenger != null || ServicesBinding.instance != null,
+      'Cannot use this MethodChannel before the binary messenger has been initialized. '
+      'This happens when you invoke platform methods before the WidgetsFlutterBinding '
+      'has been initialized. You can fix this by either calling WidgetsFlutterBinding.ensureInitialized() '
+      'before this or by passing a custom BinaryMessenger instance to MethodChannel().',
+    );
+
+    return _binaryMessenger ?? ServicesBinding.instance!.defaultBinaryMessenger;
+  }
   final BinaryMessenger? _binaryMessenger;
 
   /// Backend implementation of [invokeMethod].

packages/flutter/test/services/use_platform_channel_without_initialization_test.dart

@@ -0,0 +1,24 @@
+// Copyright 2014 The Flutter 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 'package:flutter/services.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+void main() {
+  // We need a separate test file for this test case (instead of including it
+  // in platform_channel_test.dart) since we rely on the WidgetsFlutterBinding
+  // not being initialized and we call ensureInitialized() in the other test
+  // file.
+  test('throws assertion error iff WidgetsFlutterBinding is not yet initialized', () {
+    const MethodChannel methodChannel = MethodChannel('mock');
+
+    // Ensure that accessing the binary messenger before initialization reports
+    // a helpful error message.
+    expect(() => methodChannel.binaryMessenger, throwsA(isA<AssertionError>()
+        .having((AssertionError e) => e.message, 'message', contains('WidgetsFlutterBinding.ensureInitialized()'))));
+
+    TestWidgetsFlutterBinding.ensureInitialized();
+    expect(() => methodChannel.binaryMessenger, returnsNormally);
+  });
+}