fix: fix the potential race condition when dismissing and presentatin…

…g modal (facebook#35705)

Summary:
`react-native-screens` has a bug about `UIViewControllerHierarchyInconsistency` when dismissing a `react-native` `<Modal>`. Here is the bug software-mansion/react-native-screens#944

After adding `dispatch_async` block, it solves the issue. But I do not know if this is right.

Here is the example repo https://github.com/wood1986/react-native-modal-crash

You can revert my last commit wood1986/react-native-modal-crash@86e7bc1 to reproduce the issue

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[IOS] [FIXED] - fixed the potential race condition when dismissing and presentating modal

Pull Request resolved: facebook#35705

Reviewed By: cipolleschi

Differential Revision: D42253488

Pulled By: makovkastar

fbshipit-source-id: 3e98fa9e719ecdeddeb2a367b0cd364e15136d56

React/Views/RCTModalHostViewManager.m

@@ -75,13 +75,15 @@ - (void)presentModalHostView:(RCTModalHostView *)modalHostView
       modalHostView.onShow(nil);
     }
   };
-  if (_presentationBlock) {
-    _presentationBlock([modalHostView reactViewController], viewController, animated, completionBlock);
-  } else {
-    [[modalHostView reactViewController] presentViewController:viewController
-                                                      animated:animated
-                                                    completion:completionBlock];
-  }
+  dispatch_async(dispatch_get_main_queue(), ^{
+    if (self->_presentationBlock) {
+      self->_presentationBlock([modalHostView reactViewController], viewController, animated, completionBlock);
+    } else {
+      [[modalHostView reactViewController] presentViewController:viewController
+                                                        animated:animated
+                                                      completion:completionBlock];
+    }
+  });
 }
 
 - (void)dismissModalHostView:(RCTModalHostView *)modalHostView
@@ -93,11 +95,13 @@ - (void)dismissModalHostView:(RCTModalHostView *)modalHostView
       [[self.bridge moduleForClass:[RCTModalManager class]] modalDismissed:modalHostView.identifier];
     }
   };
-  if (_dismissalBlock) {
-    _dismissalBlock([modalHostView reactViewController], viewController, animated, completionBlock);
-  } else {
-    [viewController.presentingViewController dismissViewControllerAnimated:animated completion:completionBlock];
-  }
+  dispatch_async(dispatch_get_main_queue(), ^{
+    if (self->_dismissalBlock) {
+      self->_dismissalBlock([modalHostView reactViewController], viewController, animated, completionBlock);
+    } else {
+      [viewController.presentingViewController dismissViewControllerAnimated:animated completion:completionBlock];
+    }
+  });
 }
 
 - (RCTShadowView *)shadowView