How to download EPMDocumentMaster using API? wt.util.WTRemoteException: Unable to invoke remote method;

I want to use API to download EPMDocumentMaster, see the code as below:

package com.cisco.whichen2;

import java.io.IOException;

import wt.method.RemoteAccess;

import wt.method.RemoteMethodServer;

import wt.part.WTPart;

import wt.pds.StatementSpec;

import wt.pom.PersistenceException;

import wt.pom.Transaction;

import wt.query.QuerySpec;

import wt.util.WTException;

import wt.vc.config.LatestConfigSpec;

import wt.content.ApplicationData;

import wt.content.ContentHelper;

import wt.content.ContentItem;

import wt.content.ContentServerHelper;

import wt.content.FormatContentHolder;

import wt.epm.EPMDocument;

import wt.epm.EPMDocumentMaster;

import wt.fc.QueryResult;

import wt.query.SearchCondition;

import wt.fc.PersistenceHelper;

import wt.content.ContentServerHelper;

public class PartManager implements RemoteAccess{

  public static void main(String[] args) throws Exception {

  RemoteMethodServer rms=RemoteMethodServer.getDefault();

  rms.setUserName("wcadmin");

  rms.setPassword("wcadmin");

  Class argTypes[]={String.class, String.class};

  Object argValues[]={"29-8888-01.prt","C:\\temp\\"};

  System.out.println(PartManager.class.getName());

  rms.invoke("getEPM", PartManager.class.getName(), null, argTypes, argValues);

  }

  public static void getEPM (String epmName,String path) throws Exception{

  Transaction tx=new Transaction();

  tx.start();

  EPMDocumentMaster cad = null;

  ContentItem primaryContent=null;

  if (epmName == null || epmName.trim().length()==0){

  System.out.println();

  }

  QuerySpec qs= new QuerySpec(EPMDocumentMaster.class);

  qs.appendWhere(new SearchCondition(EPMDocumentMaster.class,EPMDocumentMaster.NAME,SearchCondition.EQUAL,epmName) , new int[]{0,-1});

  System.out.println(qs);

  QueryResult qr = PersistenceHelper.manager.find((StatementSpec)qs);

  System.out.println(qr.hasMoreElements());

  while(qr.hasMoreElements()){

  cad = (EPMDocumentMaster) qr.nextElement();

  String fullName = path + cad.getCADName();

  System.out.println("downloadFile>>>EMPDocumentMaster, writeContentStream:"+fullName);

  EPMDocument latest = (EPMDocument) wt.vc.VersionControlHelper.service.allVersionsOf(cad).nextElement();

  System.out.println("Get the latest version successful");

  primaryContent=ContentHelper.getPrimary((FormatContentHolder)latest);

  System.out.println("The primary content is:");

  ContentServerHelper.service.writeContentStream((ApplicationData)primaryContent,fullName);

  }

  tx.commit();

  }

}

The error information is:

com.cisco.whichen2.PartManager

Exception in thread "main" wt.util.WTRemoteException: Unable to invoke remote method; nested exception is:

  java.rmi.ServerRuntimeException: Server exception; nested exception is:

  java.lang.NullPointerException

  at wt.method.RemoteMethodServer.invoke(RemoteMethodServer.java:788)

  at com.cisco.whichen2.PartManager.main(PartManager.java:36)

Caused by: java.lang.NullPointerException

  ... 2 more

Who can help me?

Thanks.

White