Avoid NPE in ElasticSearch 5 client due to version mismatch. (#6863)

CHANGES.md

@@ -24,6 +24,7 @@ Release Notes.
 * Add `spring-cloud-gateway-3.x` optional plugin.
 * Add `okhttp-4.x` plugin.
 * Fix NPE when thrift field is nested in plugin `thrift`
+* Fix possible NullPointerException in agent's ES plugin.
 
 #### OAP-Backend
 * BugFix: filter invalid Envoy access logs whose socket address is empty.

apm-sniffer/apm-sdk-plugin/elasticsearch-6.x-plugin/src/main/java/org/apache/skywalking/apm/plugin/elasticsearch/v6/define/RestHighLevelClientInstrumentation.java

@@ -257,4 +257,9 @@ public class RestHighLevelClientInstrumentation extends ClassEnhancePluginDefine
     public StaticMethodsInterceptPoint[] getStaticMethodsInterceptPoints() {
         return new StaticMethodsInterceptPoint[0];
     }
+
+    @Override
+    protected String[] witnessClasses() {
+        return new String[] {Constants.TASK_TRANSPORT_CHANNEL_WITNESS_CLASSES};
+    }
 }