Fix -[FlutterView focusItemsInRect:] crash (#165454)

Fixes #163999

The backing semanticsObject of the semantics container (which the
container holds a weak ref to) is deallocated after hot restart so
`[rootAccessibilityElement accessibilityElementAtIndex:0]` is returning
`nil`.

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- [ ] I added new tests to check the change I am making, or this PR is
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Fixes] where supported.
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engine/src/flutter/shell/platform/darwin/ios/framework/Source/accessibility_bridge.mm

@@ -67,7 +67,6 @@ AccessibilityBridge::AccessibilityBridge(
 AccessibilityBridge::~AccessibilityBridge() {
   [accessibility_channel_ setMessageHandler:nil];
   clearState();
-  view_controller_.viewIfLoaded.accessibilityElements = nil;
 }
 
 UIView<UITextInput>* AccessibilityBridge::textInputView() {
@@ -382,6 +381,7 @@ void AccessibilityBridge::clearState() {
   [objects_ removeAllObjects];
   previous_route_id_ = 0;
   previous_routes_.clear();
+  view_controller_.viewIfLoaded.accessibilityElements = nil;
 }
 
 }  // namespace flutter

engine/src/flutter/shell/platform/darwin/ios/framework/Source/accessibility_bridge_test.mm

@@ -2364,4 +2364,41 @@ fml::RefPtr<fml::TaskRunner> CreateNewThread(const std::string& name) {
   latch.Wait();
 }
 
+- (void)testResetsAccessibilityElementsOnHotRestart {
+  flutter::MockDelegate mock_delegate;
+  auto thread = std::make_unique<fml::Thread>("AccessibilityBridgeTest");
+  auto thread_task_runner = thread->GetTaskRunner();
+  flutter::TaskRunners runners(/*label=*/self.name.UTF8String,
+                               /*platform=*/thread_task_runner,
+                               /*raster=*/thread_task_runner,
+                               /*ui=*/thread_task_runner,
+                               /*io=*/thread_task_runner);
+  id mockFlutterView = OCMClassMock([FlutterView class]);
+  id mockFlutterViewController = OCMClassMock([FlutterViewController class]);
+  OCMStub([mockFlutterViewController viewIfLoaded]).andReturn(mockFlutterView);
+
+  fml::AutoResetWaitableEvent latch;
+  thread_task_runner->PostTask([&] {
+    auto platform_view = std::make_unique<flutter::PlatformViewIOS>(
+        /*delegate=*/mock_delegate,
+        /*rendering_api=*/mock_delegate.settings_.enable_impeller
+            ? flutter::IOSRenderingAPI::kMetal
+            : flutter::IOSRenderingAPI::kSoftware,
+        /*platform_views_controller=*/nil,
+        /*task_runners=*/runners,
+        /*worker_task_runner=*/nil,
+        /*is_gpu_disabled_sync_switch=*/std::make_shared<fml::SyncSwitch>());
+
+    platform_view->SetOwnerViewController(mockFlutterViewController);
+    platform_view->SetSemanticsEnabled(true);
+
+    OCMExpect([mockFlutterView setAccessibilityElements:[OCMArg isNil]]);
+    platform_view->OnPreEngineRestart();
+    OCMVerifyAll(mockFlutterView);
+
+    latch.Signal();
+  });
+  latch.Wait();
+}
+
 @end