Minimal fix for #1565 (#1747)

Firebase/Storage/Private/FIRStorageTask_Private.h

@@ -62,7 +62,7 @@ NS_ASSUME_NONNULL_BEGIN
 
 @property(readonly, nonatomic) GTMSessionFetcherService *fetcherService;
 
-@property(readonly) GTMSessionFetcherCompletionHandler fetcherCompletion;
+@property(readonly, copy) GTMSessionFetcherCompletionHandler fetcherCompletion;
 
 /**
  * Creates a new FIRStorageTask initialized with a FIRStorageReference and GTMSessionFetcherService.