Possible fix for convertIdToFollyDynamic crash in RCTBaseTextInputView and RCTEventDispatcher

Summary:
A crash encountered in react-native-macOS is very similar to one fixed by microsoft#489 (comment) (see discussion), and it's possible this `replacement` string also suffers from sharing the same backing store as the attributed string (maybe only when the range encompasses the entire string?) and therefore should be copied as well.

Changelog:
[iOS][Fixed] - Possible fix for convertIdToFollyDynamic crash in RCTBaseTextInputView and RCTEventDispatcher

Reviewed By: sammy-SC

Differential Revision: D38064551

fbshipit-source-id: 9c15f2a980155ab3cbb3fde79fcb93b24ee2091a

Author: christophpurrer

Libraries/Text/TextInput/RCTBaseTextInputView.m

@@ -439,7 +439,8 @@ - (NSString *)textInputShouldChangeText:(NSString *)text inRange:(NSRange)range
 
   if (_onTextInput) {
     _onTextInput(@{
-      @"text": text,
+      // We copy the string here because if it's a mutable string it may get released before we stop using it on a different thread, causing a crash.
+      @"text": [text copy],
       @"previousText": previousText,
       @"range": @{
         @"start": @(range.location),

React/CoreModules/RCTEventDispatcher.mm

@@ -86,7 +86,9 @@ - (void)sendTextEventWithType:(RCTTextEventType)type
   }];
 
   if (text) {
-    body[@"text"] = text;
+    // We copy the string here because if it's a mutable string it may get released before we dispatch the event on a
+    // different thread, causing a crash.
+    body[@"text"] = [text copy];
   }
 
   if (key) {
@@ -103,7 +105,9 @@ - (void)sendTextEventWithType:(RCTTextEventType)type
           break;
       }
     }
-    body[@"key"] = key;
+    // We copy the string here because if it's a mutable string it may get released before we dispatch the event on a
+    // different thread, causing a crash.
+    body[@"key"] = [key copy];
   }
 
   RCTComponentEvent *event = [[RCTComponentEvent alloc] initWithName:events[type] viewTag:reactTag body:body];