Fix crash when closing a window with Alt+F4 in multi-win Flutter on Windows (#161375)

Reopened from https://github.com/flutter/engine/pull/56501.

Fixes [#158450](https://github.com/flutter/flutter/issues/158450).

As mentioned in
[#158450](https://github.com/flutter/flutter/issues/158450), the crash
occurs because a destroyed object may be accessed if the window and view
have already been destroyed by the time `KeyEventCallback` is called.
This issue is not limited to the `Alt+F4` system key; it may also occur
if the window is closed using other key presses, such as pressing
`Enter` after navigating to a dialog's "Close" button.

This PR proposes a fix that checks whether the view ID is still valid
when the callback is invoked. If the view is invalid, the event is
skipped for that view.

A unit test has been added to assert that the `KeyEventCallback` is
invoked when the associated view is valid and not invoked when the view
is destroyed.

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
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engine/src/flutter/shell/platform/windows/flutter_windows_view.cc

@@ -536,12 +536,15 @@ void FlutterWindowsView::SendKey(int key,
                                  KeyEventCallback callback) {
   engine_->keyboard_key_handler()->KeyboardHook(
       key, scancode, action, character, extended, was_down,
-      [=, callback = std::move(callback)](bool handled) {
+      [engine = engine_, view_id = view_id_, key, scancode, action, character,
+       extended, was_down, callback = std::move(callback)](bool handled) {
         if (!handled) {
-          engine_->text_input_plugin()->KeyboardHook(
+          engine->text_input_plugin()->KeyboardHook(
               key, scancode, action, character, extended, was_down);
         }
-        callback(handled);
+        if (engine->view(view_id)) {
+          callback(handled);
+        }
       });
 }
 

engine/src/flutter/shell/platform/windows/flutter_windows_view_unittests.cc

@@ -306,6 +306,35 @@ TEST(FlutterWindowsViewTest, KeySequence) {
   key_event_logs.clear();
 }
 
+TEST(FlutterWindowsViewTest, KeyEventCallback) {
+  std::unique_ptr<FlutterWindowsEngine> engine = GetTestEngine();
+
+  std::unique_ptr<FlutterWindowsView> view = engine->CreateView(
+      std::make_unique<NiceMock<MockWindowBindingHandler>>());
+
+  class MockCallback {
+   public:
+    MOCK_METHOD(void, Call, ());
+  };
+
+  NiceMock<MockCallback> callback_with_valid_view;
+  NiceMock<MockCallback> callback_with_invalid_view;
+
+  auto trigger_key_event = [&](NiceMock<MockCallback>& callback) {
+    view->OnKey(kVirtualKeyA, kScanCodeKeyA, WM_KEYDOWN, 'a', false, false,
+                [&](bool) { callback.Call(); });
+  };
+
+  EXPECT_CALL(callback_with_valid_view, Call()).Times(1);
+  EXPECT_CALL(callback_with_invalid_view, Call()).Times(0);
+
+  trigger_key_event(callback_with_valid_view);
+  engine->RemoveView(view->view_id());
+  trigger_key_event(callback_with_invalid_view);
+
+  key_event_logs.clear();
+}
+
 TEST(FlutterWindowsViewTest, EnableSemantics) {
   std::unique_ptr<FlutterWindowsEngine> engine = GetTestEngine();
   EngineModifier modifier(engine.get());