奇怪的空指针异常

public void commandAction(Command cmd, Displayable d) {
        if(cmd == cmdBack){
            display.setCurrent(mainList);
        }
        else{
            Form form = new Form("kchb");
            String strWeek = listAll.getString(listAll.getSelectedIndex());           
            byte[] recordArray;
            ClassHandle ch = new ClassHandle();           
            ch.setWeek(strWeek);
            int numRecords;
            ClassInfo[] ci;
            StringItem[] si;
            try {
                RecordStore rs = RecordStore.openRecordStore("kchb", true);               
                RecordEnumeration re = rs.enumerateRecords(null, null, false);
                numRecords = re.numRecords();
                System.out.println(numRecords);
                ci = new ClassInfo[numRecords];
                si = new StringItem[numRecords];
                for(int i = 0;i<numRecords;i++){
                    recordArray = re.nextRecord();
                    ci[i] = ch.deserialize(recordArray);
                }
                rs.closeRecordStore();
                for(int i= 0;i<numRecords;i++){
                    si[i].setText("\n 课程 " + ci[i].getName()  +
                            "\n  任课教师： " + ci[i].getTeacher()
                                        + "\n  教室： " + ci[i].getRoom()
                                        + "\n 时间：    " + ci[i].getTime()
                                        +"\n 附加信息:"+ ci[i].getExtra()
                                        );
                    form.append(si[i]);
                    form.append(new Spacer(1,10));
                }
                display.setCurrent(form);
            } catch (RecordStoreFullException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (RecordStoreNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (RecordStoreNotOpenException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (InvalidRecordIDException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (RecordStoreException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }           
        }
    }