[iOS] sprinkle some null checks on BringLayersIntoView. (#55334)

A user reported hitting a null check error in this code. Add some null checks.

Author: Jonah Williams

shell/platform/darwin/ios/framework/Source/FlutterPlatformViewsTest.mm

@@ -83,6 +83,18 @@ @implementation FlutterPlatformViewsTestMockFlutterPlatformFactory
 
 @end
 
+@interface FlutterPlatformViewsTestNilFlutterPlatformFactory : NSObject <FlutterPlatformViewFactory>
+@end
+
+@implementation FlutterPlatformViewsTestNilFlutterPlatformFactory
+- (NSObject<FlutterPlatformView>*)createWithFrame:(CGRect)frame
+                                   viewIdentifier:(int64_t)viewId
+                                        arguments:(id _Nullable)args {
+  return nil;
+}
+
+@end
+
 namespace flutter {
 namespace {
 class FlutterPlatformViewsTestMockPlatformViewDelegate : public PlatformView::Delegate {
@@ -3692,6 +3704,73 @@ - (void)testOnlyPlatformViewsAreRemovedWhenReset {
   XCTAssertEqual(flutterView.subviews.firstObject, someView);
 }
 
+- (void)testNilPlatformViewDoesntCrash {
+  flutter::FlutterPlatformViewsTestMockPlatformViewDelegate mock_delegate;
+
+  flutter::TaskRunners runners(/*label=*/self.name.UTF8String,
+                               /*platform=*/GetDefaultTaskRunner(),
+                               /*raster=*/GetDefaultTaskRunner(),
+                               /*ui=*/GetDefaultTaskRunner(),
+                               /*io=*/GetDefaultTaskRunner());
+  auto flutterPlatformViewsController = std::make_shared<flutter::PlatformViewsController>();
+  flutterPlatformViewsController->SetTaskRunner(GetDefaultTaskRunner());
+  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=*/flutterPlatformViewsController,
+      /*task_runners=*/runners,
+      /*worker_task_runner=*/nil,
+      /*is_gpu_disabled_jsync_switch=*/std::make_shared<fml::SyncSwitch>());
+
+  FlutterPlatformViewsTestNilFlutterPlatformFactory* factory =
+      [[FlutterPlatformViewsTestNilFlutterPlatformFactory alloc] init];
+  flutterPlatformViewsController->RegisterViewFactory(
+      factory, @"MockFlutterPlatformView",
+      FlutterPlatformViewGestureRecognizersBlockingPolicyEager);
+  FlutterResult result = ^(id result) {
+  };
+  flutterPlatformViewsController->OnMethodCall(
+      [FlutterMethodCall
+          methodCallWithMethodName:@"create"
+                         arguments:@{@"id" : @2, @"viewType" : @"MockFlutterPlatformView"}],
+      result);
+  UIView* flutterView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 500, 500)];
+  flutterPlatformViewsController->SetFlutterView(flutterView);
+
+  // Create embedded view params
+  flutter::MutatorsStack stack;
+  // Layer tree always pushes a screen scale factor to the stack
+  SkMatrix screenScaleMatrix =
+      SkMatrix::Scale([UIScreen mainScreen].scale, [UIScreen mainScreen].scale);
+  stack.PushTransform(screenScaleMatrix);
+  // Push a translate matrix
+  SkMatrix translateMatrix = SkMatrix::Translate(100, 100);
+  stack.PushTransform(translateMatrix);
+  SkMatrix finalMatrix;
+  finalMatrix.setConcat(screenScaleMatrix, translateMatrix);
+
+  auto embeddedViewParams =
+      std::make_unique<flutter::EmbeddedViewParams>(finalMatrix, SkSize::Make(300, 300), stack);
+
+  flutterPlatformViewsController->PrerollCompositeEmbeddedView(2, std::move(embeddedViewParams));
+
+  // SKSurface is required if the root FlutterView is present.
+  const SkImageInfo image_info = SkImageInfo::MakeN32Premul(1000, 1000);
+  sk_sp<SkSurface> mock_sk_surface = SkSurfaces::Raster(image_info);
+  flutter::SurfaceFrame::FramebufferInfo framebuffer_info;
+  auto mock_surface = std::make_unique<flutter::SurfaceFrame>(
+      std::move(mock_sk_surface), framebuffer_info,
+      [](const flutter::SurfaceFrame& surface_frame, flutter::DlCanvas* canvas) { return true; },
+      [](const flutter::SurfaceFrame& surface_frame) { return true; },
+      /*frame_size=*/SkISize::Make(800, 600));
+
+  flutterPlatformViewsController->SubmitFrame(nullptr, nullptr, std::move(mock_surface));
+
+  XCTAssertEqual(flutterView.subviews.count, 1u);
+}
+
 - (void)testFlutterTouchInterceptingViewLinksToAccessibilityContainer {
   FlutterTouchInterceptingView* touchInteceptorView = [[FlutterTouchInterceptingView alloc] init];
   NSObject* container = [[NSObject alloc] init];

shell/platform/darwin/ios/framework/Source/platform_views_controller.mm

@@ -797,13 +797,18 @@ bool ClipRRectContainsPlatformViewBoundingRect(const SkRRect& clip_rrect,
   NSMutableArray* desired_platform_subviews = [NSMutableArray array];
   for (int64_t platform_view_id : composition_order) {
     UIView* platform_view_root = platform_views_[platform_view_id].root_view.get();
-    [desired_platform_subviews addObject:platform_view_root];
+    if (platform_view_root != nil) {
+      [desired_platform_subviews addObject:platform_view_root];
+    }
 
     auto maybe_layer_data = layer_map.find(platform_view_id);
     if (maybe_layer_data != layer_map.end()) {
-      [desired_platform_subviews addObject:maybe_layer_data->second.layer->overlay_view_wrapper];
+      auto view = maybe_layer_data->second.layer->overlay_view_wrapper;
+      if (view != nil) {
+        [desired_platform_subviews addObject:view];
+        previous_composition_order_.push_back(platform_view_id);
+      }
     }
-    previous_composition_order_.push_back(platform_view_id);
   }
 
   NSSet* desired_platform_subviews_set = [NSSet setWithArray:desired_platform_subviews];