Fix NullPointerException reported from Third Eye

Change-Id: I9acd0ef02aa1d659b6f166d27013d44a390c561d
diff --git a/src/main/java/com/googlesource/gerrit/plugins/automerger/CherryPickChangeCreator.java b/src/main/java/com/googlesource/gerrit/plugins/automerger/CherryPickChangeCreator.java
index 6eac9f6..c112ae8 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/automerger/CherryPickChangeCreator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/automerger/CherryPickChangeCreator.java
@@ -149,8 +149,8 @@
     List<ChangeInfo> changes =
         ChangeUtils.getChangesInTopicAndBranch(gApi, currentTopic, sdsChangeInput.downstreamBranch);
     for (ChangeInfo change : changes) {
-      if(change.cherryPickOfChange.equals(sdsChangeInput.changeNumber)
-          && change.cherryPickOfPatchSet == sdsChangeInput.patchsetNumber) {
+      if(change.cherryPickOfChange != null && change.cherryPickOfChange.equals(sdsChangeInput.changeNumber)
+          && change.cherryPickOfPatchSet != null && change.cherryPickOfPatchSet.equals(sdsChangeInput.patchsetNumber)) {
         return true;
       }
     }