为什么我的程序不能向arraylist<>里面添加对象

程序一运行的红色部分就出错了public ArrayList<Nokia5800Skill> selectAll(){
ArrayList<Nokia5800Skill> skills=null;
try{
Connection con=DBConnection.getConnection();
String sql="select * from nokia5800Skill";
PreparedStatement pstmt=con.prepareStatement(sql);
ResultSet rs=pstmt.executeQuery();
while(rs.next()){
Nokia5800Skill skill=new Nokia5800Skill();
skill.setId(rs.getInt(1));
skill.setTitle(rs.getString(2));
skill.setPublishTime(rs.getDate(3));
skill.setContext(rs.getString(4));
skill.setClicks(rs.getInt(5));
skills.add(skill); }
}catch(SQLException se){
se.printStackTrace();
}
return skills;
}2009-6-2 23:24:12 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at BasicImplement.Nokia5800SkillImplement.selectAll(Nokia5800SkillImplement.java:37)
at org.apache.jsp.index_jsp._jspService(index_jsp.java:104)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:337)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:266)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:803)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:175)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:286)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:844)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:447)
at java.lang.Thread.run(Unknown Source)