Fix multiline TextInput crash when inserting/removing lots of text (#…

…29307)

Summary:
Multiline `TextInput` can crash when really long texts are inserted and removed quickly. This is caused by the fact that [`-[NSAttributedString string]`](https://developer.apple.com/documentation/foundation/nsattributedstring/1412616-string?language=objc) doesn't really return a copy, and may mutate the string while it is being used by `convertIdToFollyDynamic`. See microsoft#489 (comment) for more details on the issue.

This issue was originally reported in microsoft#486 and was fixed in microsoft#489.

## Changelog

[iOS] [Fixed] - Fix multiline TextInput crash when inserting/removing lots of text

Pull Request resolved: #29307

Test Plan:
1. Open RNTester > TextInput
2. Search for a multiline example
3. Copy some large text and paste it into the text input view
4. Remove some (or all) text
5. Repeat steps 3-4

Reviewed By: shergin

Differential Revision: D22488854

Pulled By: JoshuaGross

fbshipit-source-id: 6fab7818d68538450d93460361ff5934caf86c10

Libraries/Text/TextInput/RCTBaseTextInputView.m

@@ -333,7 +333,7 @@ - (void)textInputDidBeginEditing
 
   [_eventDispatcher sendTextEventWithType:RCTTextEventTypeFocus
                                  reactTag:self.reactTag
-                                     text:self.backedTextInputView.attributedText.string
+                                     text:[self.backedTextInputView.attributedText.string copy]
                                       key:nil
                                eventCount:_nativeEventCount];
 }
@@ -347,13 +347,13 @@ - (void)textInputDidEndEditing
 {
   [_eventDispatcher sendTextEventWithType:RCTTextEventTypeEnd
                                  reactTag:self.reactTag
-                                     text:self.backedTextInputView.attributedText.string
+                                     text:[self.backedTextInputView.attributedText.string copy]
                                       key:nil
                                eventCount:_nativeEventCount];
 
   [_eventDispatcher sendTextEventWithType:RCTTextEventTypeBlur
                                  reactTag:self.reactTag
-                                     text:self.backedTextInputView.attributedText.string
+                                     text:[self.backedTextInputView.attributedText.string copy]
                                       key:nil
                                eventCount:_nativeEventCount];
 }
@@ -367,7 +367,7 @@ - (BOOL)textInputShouldReturn
   // (no connection to any specific "submitting" process).
   [_eventDispatcher sendTextEventWithType:RCTTextEventTypeSubmit
                                  reactTag:self.reactTag
-                                     text:self.backedTextInputView.attributedText.string
+                                     text:[self.backedTextInputView.attributedText.string copy]
                                       key:nil
                                eventCount:_nativeEventCount];
 
@@ -422,7 +422,7 @@ - (NSString *)textInputShouldChangeText:(NSString *)text inRange:(NSRange)range
     }
   }
 
-  NSString *previousText = backedTextInputView.attributedText.string ?: @"";
+  NSString *previousText = [backedTextInputView.attributedText.string copy] ?: @"";
 
   if (range.location + range.length > backedTextInputView.attributedText.string.length) {
     _predictedText = backedTextInputView.attributedText.string;
@@ -468,7 +468,7 @@ - (void)textInputDidChange
 
   if (_onChange) {
     _onChange(@{
-       @"text": self.attributedText.string,
+       @"text": [self.attributedText.string copy],
        @"target": self.reactTag,
        @"eventCount": @(_nativeEventCount),
     });