java.lang.NullPointerException
java.text.SimpleDateFormat.parse(SimpleDateFormat.java:1235)
java.text.DateFormat.parse(DateFormat.java:335)
www.bttc.manager.case_historyservlet.AddCase_History.doPost(AddCase_History.java:55)
javax.servlet.http.HttpServlet.service(HttpServlet.java:710)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
出现这个错误是什么意思?