要在课程中心，做《生死学》这门选修课的作业，可是两三天了，都不是打不开。出现一些这样的东西。有同学也是遇到 这样的情况吗？怎么办才好？？
500 Servlet Exception
java.lang.NullPointerException
at com.collegesoft.db.SQLExecutor.queryRowSet(SQLExecutor.java:89)
at com.collegesoft.tools.SqlHelper.iterate(SqlHelper.java:156)
at _index._homepage._index_0download__jsp._jspService(/course_center/index/homepage/index_download.jsp:26)
at com.caucho.jsp.JavaPage.service(JavaPage.java:75)
at com.caucho.jsp.Page.subservice(Page.java:506)
at com.caucho.server.http.FilterChainPage.doFilter(FilterChainPage.java:182)
at com.caucho.server.http.Invocation.service(Invocation.java:315)
at com.caucho.server.http.QRequestDispatcher.include(QRequestDispatcher.java:365)
at com.caucho.server.http.QRequestDispatcher