ReceiveCommits: Avoid potential NPE in autoCloseChanges()

Change-Id: If201d1eff57008f8ae3bacdf078c6f8ce8448fb7
diff --git a/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java b/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java
index 49080f7..053a60c 100644
--- a/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java
+++ b/gerrit-server/src/main/java/com/google/gerrit/server/git/ReceiveCommits.java
@@ -2393,6 +2393,9 @@
               closeChange(cmd, PatchSet.Id.fromRef(ref.getName()), c);
           closeProgress.update(1);
           if (closedChange != null) {
+            if (byKey == null) {
+              byKey = openChangesByKey(branch);
+            }
             byKey.remove(closedChange);
           }
         }