空指针异常

public List<Message> queryMessageList(String command,String description ){
  DBAccess dbAccess = new DBAccess();
  SqlSession sqlSession = null;
  List<Message> list = new ArrayList<Message>();
  try {
   dbAccess.getSqlSession();
   //通过sqlSession执行sql语句
   list = sqlSession.selectList("Message.queryMessageList");//空指针异常
  } catch (IOException e) {
   e.printStackTrace();
  } finally {
   if(sqlSession!=null){
    sqlSession.close();
   }
  }
  return list;
  
 }