Null Pointer Exception when testing wsdl2Apex class

I am trying to write a simple apex test class for the wsdl2apex file that i generated, but for some reason I am getting a null pointer exception in the test method which mocks the apex callout. I developed this with reference to the documentation at - https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_callouts_wsdl2apex_testing.htm The problem is in below class at line respElement.orderResponse.CorrelationID = '23145'; where orderResponse is an inner class. Here is my apex test classes:

@isTest
global class GcsmsWebmethodsMock implements WebServiceMock{

global void doInvoke(Object stub,
           Object request,
           Map<String, Object> response,
           String endpoint,
           String soapAction,
           String requestName,
           String responseNS,
           String responseName,
           String responseType){

  gcsmsWebmethodsNaJnjComMu650Gcsms.processOrderResponse respElement = new gcsmsWebmethodsNaJnjComMu650Gcsms.processOrderResponse();
  respElement.orderResponse.CorrelationID = '23145';
  response.put('response_x', respElement);


}

}

Class which calls webservice:

public class WebSvcCallout {
    public static gcsmsWebmethodsNaJnjComMu650Gcsms.orderResponse callEchoString(String input) {
    gcsmsWebmethodsNaJnjComMu650Gcsms.MU650_GCSMS_Order_v1_webservices_order_wsd_pro_Port sample = new gcsmsWebmethodsNaJnjComMu650Gcsms.MU650_GCSMS_Order_v1_webservices_order_wsd_pro_Port();
        sample.endpoint_x = 'http://api.salesforce.com/foo/bar';

        gcsmsWebmethodsNaJnjComMu650Gcsms.orderInput req = new gcsmsWebmethodsNaJnjComMu650Gcsms.orderInput();
        // This invokes the EchoString method in the generated class
        gcsmsWebmethodsNaJnjComMu650Gcsms.orderResponse res = sample.processOrder(req);

        return res;
    }   
}

The test class which calls the web service. here I get null pointer exception - attempted to de-reference null object(please see the comment below at which line)

@isTest
private class WebSvcCalloutTest {
    @isTest static void testEchoString() {              
        // This causes a fake response to be generated
        Test.setMock(WebServiceMock.class, new GcsmsWebmethodsMock());

        // Call the method that invokes a callout
        gcsmsWebmethodsNaJnjComMu650Gcsms.orderResponse output = WebSvcCallout.callEchoString('Hello World!'); // null pointer exception - //attempted to de-reference null object

        // Verify that a fake result is returned
        System.assertEquals('23145', output.CorrelationID); 
    }
}