[Windows] Fix crash of TextInputPlugin on empty model (#24293)

ci/licenses_golden/licenses_flutter

@@ -1516,6 +1516,7 @@ FILE: ../../../flutter/shell/platform/windows/text_input_manager_win32.h
 FILE: ../../../flutter/shell/platform/windows/text_input_plugin.cc
 FILE: ../../../flutter/shell/platform/windows/text_input_plugin.h
 FILE: ../../../flutter/shell/platform/windows/text_input_plugin_delegate.h
+FILE: ../../../flutter/shell/platform/windows/text_input_plugin_unittest.cc
 FILE: ../../../flutter/shell/platform/windows/win32_dpi_utils.cc
 FILE: ../../../flutter/shell/platform/windows/win32_dpi_utils.h
 FILE: ../../../flutter/shell/platform/windows/win32_dpi_utils_unittests.cc

shell/platform/windows/BUILD.gn

@@ -199,6 +199,7 @@ executable("flutter_windows_unittests") {
       "testing/mock_window_binding_handler.h",
       "testing/win32_flutter_window_test.cc",
       "testing/win32_flutter_window_test.h",
+      "text_input_plugin_unittest.cc",
       "win32_dpi_utils_unittests.cc",
       "win32_flutter_window_unittests.cc",
       "win32_window_proc_delegate_manager_unittests.cc",

shell/platform/windows/key_event_handler_unittests.cc

@@ -15,11 +15,12 @@
 namespace flutter {
 namespace testing {
 
+namespace {
 static constexpr char kScanCodeKey[] = "scanCode";
 static constexpr int kHandledScanCode = 20;
 static constexpr int kUnhandledScanCode = 21;
 
-std::unique_ptr<std::vector<uint8_t>> CreateResponse(bool handled) {
+static std::unique_ptr<std::vector<uint8_t>> CreateResponse(bool handled) {
   auto response_doc =
       std::make_unique<rapidjson::Document>(rapidjson::kObjectType);
   auto& allocator = response_doc->GetAllocator();
@@ -27,6 +28,8 @@ std::unique_ptr<std::vector<uint8_t>> CreateResponse(bool handled) {
   return JsonMessageCodec::GetInstance().EncodeMessage(*response_doc);
 }
 
+}  // namespace
+
 TEST(KeyEventHandlerTest, KeyboardHookHandling) {
   auto handled_message = CreateResponse(true);
   auto unhandled_message = CreateResponse(false);

shell/platform/windows/text_input_plugin.cc

@@ -103,11 +103,17 @@ TextInputPlugin::TextInputPlugin(flutter::BinaryMessenger* messenger,
 TextInputPlugin::~TextInputPlugin() = default;
 
 void TextInputPlugin::ComposeBeginHook() {
+  if (active_model_ == nullptr) {
+    return;
+  }
   active_model_->BeginComposing();
   SendStateUpdate(*active_model_);
 }
 
 void TextInputPlugin::ComposeEndHook() {
+  if (active_model_ == nullptr) {
+    return;
+  }
   active_model_->CommitComposing();
   active_model_->EndComposing();
   SendStateUpdate(*active_model_);
@@ -115,6 +121,9 @@ void TextInputPlugin::ComposeEndHook() {
 
 void TextInputPlugin::ComposeChangeHook(const std::u16string& text,
                                         int cursor_pos) {
+  if (active_model_ == nullptr) {
+    return;
+  }
   active_model_->AddText(text);
   cursor_pos += active_model_->composing_range().base();
   active_model_->UpdateComposingText(text);

shell/platform/windows/text_input_plugin_unittest.cc

+// Copyright 2013 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.
+#include "flutter/shell/platform/windows/text_input_plugin.h"
+
+#include <rapidjson/document.h>
+#include <memory>
+
+#include "flutter/shell/platform/common/json_message_codec.h"
+#include "flutter/shell/platform/windows/flutter_windows_view.h"
+#include "flutter/shell/platform/windows/testing/test_binary_messenger.h"
+#include "gmock/gmock.h"
+#include "gtest/gtest.h"
+
+namespace flutter {
+namespace testing {
+
+namespace {
+static constexpr char kScanCodeKey[] = "scanCode";
+static constexpr int kHandledScanCode = 20;
+static constexpr int kUnhandledScanCode = 21;
+
+static std::unique_ptr<std::vector<uint8_t>> CreateResponse(bool handled) {
+  auto response_doc =
+      std::make_unique<rapidjson::Document>(rapidjson::kObjectType);
+  auto& allocator = response_doc->GetAllocator();
+  response_doc->AddMember("handled", handled, allocator);
+  return JsonMessageCodec::GetInstance().EncodeMessage(*response_doc);
+}
+
+class EmptyTextInputPluginDelegate : public TextInputPluginDelegate {
+ public:
+  // Notifies delegate that the cursor position has changed.
+  void OnCursorRectUpdated(const Rect& rect) {}
+};
+}  // namespace
+
+TEST(TextInputPluginTest, TextMethodsWorksWithEmptyModel) {
+  auto handled_message = CreateResponse(true);
+  auto unhandled_message = CreateResponse(false);
+  int received_scancode = 0;
+
+  TestBinaryMessenger messenger(
+      [&received_scancode, &handled_message, &unhandled_message](
+          const std::string& channel, const uint8_t* message,
+          size_t message_size, BinaryReply reply) {});
+  EmptyTextInputPluginDelegate delegate;
+
+  int redispatch_scancode = 0;
+  TextInputPlugin handler(&messenger, &delegate);
+
+  handler.KeyboardHook(nullptr, VK_RETURN, 100, WM_KEYDOWN, '\n', false);
+  handler.ComposeBeginHook();
+  std::u16string text;
+  text.push_back('\n');
+  handler.ComposeChangeHook(text, 1);
+  handler.ComposeEndHook();
+
+  // Passes if it did not crash
+}
+
+}  // namespace testing
+}  // namespace flutter