Fix nil pointer issue

Author: Shaddoll

src/main/java/com/uber/cadence/internal/common/WorkflowExecutionUtils.java

@@ -226,7 +226,7 @@ private static HistoryEvent getInstanceCloseEvent(
 
       pageToken = response.getNextPageToken();
       History history = response.getHistory();
-      if (history != null && history.getEvents().size() > 0) {
+      if (history != null && history.getEvents() != null && history.getEvents().size() > 0) {
         event = history.getEvents().get(0);
         if (!isWorkflowExecutionCompletedEvent(event)) {
           throw new RuntimeException("Last history event is not completion event: " + event);