[ABT] Fix race crash (#16148)

Co-authored-by: Nick Cooke <36927374+ncooke3@users.noreply.github.com>

Author: paulb777

FirebaseABTesting/CHANGELOG.md

@@ -1,3 +1,7 @@
+# Unreleased
+- [fixed] Fixed a race condition that could lead to a crash in ABTesting when
+  updating experiments. (#16145)
+
 # 10.16.0
 - [fixed] Fix crash caused by empty experiment payload. (#11873)
 

FirebaseABTesting/Sources/FIRExperimentController.m

@@ -165,14 +165,15 @@ - (void)updateExperimentsWithServiceOrigin:(NSString *)origin
                                   payloads:(NSArray<NSData *> *)payloads
                          completionHandler:
                              (nullable void (^)(NSError *_Nullable error))completionHandler {
+  NSArray<NSData *> *payloadsCopy = [payloads copy];
   FIRExperimentController *__weak weakSelf = self;
   dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
     FIRExperimentController *strongSelf = weakSelf;
     [strongSelf updateExperimentConditionalUserPropertiesWithServiceOrigin:origin
                                                                     events:events
                                                                     policy:policy
                                                              lastStartTime:lastStartTime
-                                                                  payloads:payloads
+                                                                  payloads:payloadsCopy
                                                          completionHandler:completionHandler];
   });
 }