fix crash SemanticsObject dealloc and access the children

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

@@ -372,6 +372,7 @@ - (void)privateSetParent:(SemanticsObject*)parent;
 @implementation SemanticsObject {
   fml::scoped_nsobject<SemanticsObjectContainer> _container;
   NSMutableArray<SemanticsObject*>* _children;
+  BOOL _inDealloc;
 }
 
 #pragma mark - Override base class designated initializers
@@ -413,6 +414,7 @@ - (void)dealloc {
   _parent = nil;
   _container.get().semanticsObject = nil;
   [_platformViewSemanticsContainer release];
+  _inDealloc = YES;
   [super dealloc];
 }
 
@@ -686,6 +688,14 @@ - (void)setAccessibilityContainer:(id)container {
 }
 
 - (id)accessibilityContainer {
+  if (_inDealloc) {
+    // In iOS9, `accessibilityContainer` will be called by `[UIAccessibilityElementSuperCategory
+    // dealloc]` during `[super dealloc]`. And will crash when accessing `_children` which has
+    // called `[_children release]` in `[SemanticsObject dealloc]`.
+    // https://github.com/flutter/flutter/issues/87247
+    return nil;
+  }
+
   if (![self isAccessibilityBridgeAlive]) {
     return nil;
   }

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

@@ -644,6 +644,20 @@ - (void)testFlutterSwitchSemanticsObjectMatchesUISwitch {
   XCTAssertEqual(object.accessibilityValue, nativeSwitch.accessibilityValue);
 }
 
+- (void)testSemanticsObjectDeallocated {
+  fml::WeakPtrFactory<flutter::AccessibilityBridgeIos> factory(
+      new flutter::MockAccessibilityBridge());
+  fml::WeakPtr<flutter::AccessibilityBridgeIos> bridge = factory.GetWeakPtr();
+  SemanticsObject* parent = [[SemanticsObject alloc] initWithBridge:bridge uid:0];
+  SemanticsObject* child = [[SemanticsObject alloc] initWithBridge:bridge uid:1];
+  parent.children = @[ child ];
+  // Validate SemanticsObject deallocation does not crash.
+  // https://github.com/flutter/flutter/issues/66032
+  __weak SemanticsObject* weakObject = parent;
+  parent = nil;
+  XCTAssertNil(weakObject);
+}
+
 - (void)testFlutterSemanticsObjectReturnsNilContainerWhenBridgeIsNotAlive {
   FlutterSemanticsObject* parentObject;
   FlutterSemanticsObject* delegate;