Update RCTModalHostViewManager.m (#29745)

Summary:
Fix a issue that RCTModalHostView can't be dismissed while being presented

Steps To Reproduce
A native modal presented view controller is being dismissed before the first screen shows.
A RCTModalHostView is presented when the first screen shows.
The RCTModalHostView will be dismissed after an asynchronous callback.
If the callback was called before the completion of the presenting animation, the RCTModalHostView will not be dismissed.

## Changelog

[iOS] [Fixed] - Fix that RCTModalHostView can't be dismissed while being presented

Pull Request resolved: #29745

Reviewed By: shergin

Differential Revision: D23566487

Pulled By: sammy-SC

fbshipit-source-id: bd95f200b79fa75e2387e402091d58c0f538759c

React/Views/RCTModalHostViewManager.m

@@ -48,6 +48,8 @@ - (void)insertReactSubview:(id<RCTComponent>)subview atIndex:(NSInteger)atIndex
 
 @interface RCTModalHostViewManager () <RCTModalHostViewInteractor>
 
+@property (nonatomic, copy) dispatch_block_t dismissWaitingBlock;
+
 @end
 
 @implementation RCTModalHostViewManager {
@@ -79,9 +81,16 @@ - (void)presentModalHostView:(RCTModalHostView *)modalHostView
   if (_presentationBlock) {
     _presentationBlock([modalHostView reactViewController], viewController, animated, completionBlock);
   } else {
+    __weak typeof(self) weakself = self;
     [[modalHostView reactViewController] presentViewController:viewController
                                                       animated:animated
-                                                    completion:completionBlock];
+                                                    completion:^{
+                                                      !completionBlock ?: completionBlock();
+                                                      __strong typeof(weakself) strongself = weakself;
+                                                      !strongself.dismissWaitingBlock
+                                                          ?: strongself.dismissWaitingBlock();
+                                                      strongself.dismissWaitingBlock = nil;
+                                                    }];
   }
 }
 
@@ -92,7 +101,13 @@ - (void)dismissModalHostView:(RCTModalHostView *)modalHostView
   if (_dismissalBlock) {
     _dismissalBlock([modalHostView reactViewController], viewController, animated, nil);
   } else {
-    [viewController.presentingViewController dismissViewControllerAnimated:animated completion:nil];
+    self.dismissWaitingBlock = ^{
+      [viewController.presentingViewController dismissViewControllerAnimated:animated completion:nil];
+    };
+    if (viewController.presentingViewController) {
+      self.dismissWaitingBlock();
+      self.dismissWaitingBlock = nil;
+    }
   }
 }