Hi,
i am getting null object error while trying to test  the Mock of a webservice callout.
following is my WSDL2Apex class generated :

//Generated by wsdl2apex

public class parksServices {
    public class byCountryResponse {
        public String[] return_x;
        private String[] return_x_type_info = new String[]{'return','http://parks.services/',null,'0','-1','false'};
        private String[] apex_schema_type_info = new String[]{'http://parks.services/','false','false'};
        private String[] field_order_type_info = new String[]{'return_x'};
    }
    public class byCountry {
        public String arg0;
        private String[] arg0_type_info = new String[]{'arg0','http://parks.services/',null,'0','1','false'};
        private String[] apex_schema_type_info = new String[]{'http://parks.services/','false','false'};
        private String[] field_order_type_info = new String[]{'arg0'};
    }
    public class ParksImplPort {
        public String endpoint_x = 'https://th-apex-soap-service.herokuapp.com/service/parks';
        public Map<String,String> inputHttpHeaders_x;
        public Map<String,String> outputHttpHeaders_x;
        public String clientCertName_x;
        public String clientCert_x;
        public String clientCertPasswd_x;
        public Integer timeout_x;
        private String[] ns_map_type_info = new String[]{'http://parks.services/', 'parksServices'};
        public String[] byCountry(String arg0) {
            parksServices.byCountry request_x = new parksServices.byCountry();
            request_x.arg0 = arg0;
            parksServices.byCountryResponse response_x;
            Map<String, parksServices.byCountryResponse> response_map_x = new Map<String, parksServices.byCountryResponse>();
            response_map_x.put('response_x', response_x);
            WebServiceCallout.invoke(
              this,
              request_x,
              response_map_x,
              new String[]{endpoint_x,
              '',
              'http://parks.services/',
              'byCountry',
              'http://parks.services/',
              'byCountryResponse',
              'parksServices.byCountryResponse'}
            );
            response_x = response_map_x.get('response_x');
            return response_x.return_x;
        }
    }
}

My Main Callout class :
public class Parkstub {
   
    public static list<string> parkstub1( string a)
    {   
       parksServices.ParksImplPort parklist = new parksServices.ParksImplPort();
       return parklist.byCountry(a);
       
      }

}


Mock Class

@isTest
global class ParkstubMock 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) {
        // start - specify the response you want to send
         parksServices.byCountryResponse response_x1 = new parksServices.byCountryResponse();
               response_x1.return_x.add('Yellowstone');
               response_x1.return_x.add('Mackinac National Park');
               response_x1.return_x.add('Yosemite');
       // response_x.return_x = 3.0;
        // end
        response.put('response_x', response_x1); 
   }
}

Test Class :
@isTest
public class ParkStubTest {
     @isTest static void testCallout1() {              
        // This causes a fake response to be generated
        Test.setMock(WebServiceMock.class, new ParkstubMock());
        // Call the method that invokes a callout
        string a = 'United States';
       // Double y = 2.0;
        list<string> result = Parkstub.parkstub1(a);
        // Verify that a fake result is returned
        //System.assertEquals(3.0, result); 
    }

}

While running the test class its giving me the error of attempt to dereference Null object  for Mockclass.How should i create the instance for the list of string?
Please help me.

Thanks,
Bhaskar