From 7422f6218291c66f850c81a1c9ec68516c936e87 Mon Sep 17 00:00:00 2001
From: Morgan Chen <morganchen12@gmail.com>
Date: Fri, 2 Feb 2018 11:22:50 -0800
Subject: fix null block execution crash

---
 Firestore/Source/Core/FSTFirestoreClient.mm | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/Firestore/Source/Core/FSTFirestoreClient.mm b/Firestore/Source/Core/FSTFirestoreClient.mm
index fff644d..5986b5b 100644
--- a/Firestore/Source/Core/FSTFirestoreClient.mm
+++ b/Firestore/Source/Core/FSTFirestoreClient.mm
@@ -253,9 +253,11 @@ NS_ASSUME_NONNULL_BEGIN
             completion:(nullable FSTVoidErrorBlock)completion {
   [self.workerDispatchQueue dispatchAsync:^{
     if (mutations.count == 0) {
-      [self.userDispatchQueue dispatchAsync:^{
-        completion(nil);
-      }];
+      if (completion) {
+        [self.userDispatchQueue dispatchAsync:^{
+          completion(nil);
+        }];
+      }
     } else {
       [self.syncEngine writeMutations:mutations
                            completion:^(NSError *error) {
-- 
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