fixe bug

@@ -83,7 +83,9 @@ public class DNSMonitorServiceImpl implements IDNSMonitorService {
                     String result = restTemplateNoEncode.doGet(buildDNSUrl(host, uid));
 
                     DEBUG.info("query dns result {} from host {}", result, host);
-
+                    if (result == null){
+                        continue;
+                    }
                     List<String> ipModelList = parseIPModels(result);
 
                     if (null == ipModelList) {