Fix crash inside RCTRedBox when trying to present same UIViewController twice

Summary:
Calling `-[RCTRedBox showErrorMessage]` twice causes a crash

We used `-[UIViewController isBeingPresented]` to tell whether view controller is already presented.
But from the documentation:

> A Boolean value indicating whether the view controller is being presented.

Source: https://developer.apple.com/documentation/uikit/uiviewcontroller/2097564-beingpresented?language=objc#

 ---
So this means that if you present it, wait until presentation animation is finished and then call `-[RCTRedBox showErrorMessage]` again, following exception will be thrown.

```
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Application tried to present modally an active controller <UIViewController: 0x7fc33e422f50>.'
```

Changelog: Fix crash caused by presenting view controller twice from RCTRedBox

Reviewed By: PeteTheHeat

Differential Revision: D20946645

fbshipit-source-id: 763066e37db4e56efb0118b2e7867ad0724bae81

Author: sammy-SC

React/CoreModules/RCTRedBox.mm

@@ -235,10 +235,11 @@ - (void)showErrorMessage:(NSString *)message
   // Remove ANSI color codes from the message
   NSString *messageWithoutAnsi = [self stripAnsi:message];
 
+  BOOL isRootViewControllerPresented = self.rootViewController.presentingViewController != nil;
   // Show if this is a new message, or if we're updating the previous message
-  BOOL isNew = !self.rootViewController.isBeingPresented && !isUpdate;
+  BOOL isNew = !isRootViewControllerPresented && !isUpdate;
   BOOL isUpdateForSameMessage = !isNew &&
-      (self.rootViewController.isBeingPresented && isUpdate &&
+      (isRootViewControllerPresented && isUpdate &&
        ((errorCookie == -1 && [_lastErrorMessage isEqualToString:messageWithoutAnsi]) ||
         (errorCookie == _lastErrorCookie)));
   if (isNew || isUpdateForSameMessage) {
@@ -250,7 +251,7 @@ - (void)showErrorMessage:(NSString *)message
 
     [_stackTraceTableView reloadData];
 
-    if (!self.rootViewController.isBeingPresented) {
+    if (!isRootViewControllerPresented) {
       [_stackTraceTableView scrollToRowAtIndexPath:[NSIndexPath indexPathForRow:0 inSection:0]
                                   atScrollPosition:UITableViewScrollPositionTop
                                           animated:NO];