oncreateDialog时报了空指针异常，求大神指导一下，我也找不到原因所在

Question

报这个异常，我也搜了相关资料，很多人代码里最后default也返回null，为什么我报错呀？求大神知道，代码在最下面
java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2378)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2430)
at android.app.ActivityThread.access$600(ActivityThread.java:166)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1374)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5400)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:525)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:853)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)
at dalvik.system.NativeStart.main(Native Method)
Caused by: java.lang.NullPointerException
at com.netschool.mock.MockListActivity.onCreateDialog(Unknown Source)
at android.app.Activity.onCreateDialog(Activity.java:3089)
at android.app.Activity.createDialog(Activity.java:1072)
at android.app.Activity.restoreManagedDialogs(Activity.java:1061)
at android.app.Activity.performRestoreInstanceState(Activity.java:1007)
at android.app.Instrumentation.callActivityOnRestoreInstanceState(Instrumentation.java:1202)
at com.lbe.security.service.core.client.internal.InstrumentationDelegate.callActivityOnRestoreInstanceState(InstrumentationDelegate.java:130)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2356)
... 11 more

代码

 @Override
    protected Dialog onCreateDialog(int id) {
        m_pickerDialog = null;
        switch (id) {
        case 0:

            m_pickerDialog = new DatePickerDialog_WithoutDate(this, new DatePickerDialog.OnDateSetListener() {
                @Override
                public void onDateSet(DatePicker view, int year, int monthOfYear, int dayOfMonth) {
                    Message msg = Message.obtain();
                    msg.arg1 = year;
                    msg.arg2 = monthOfYear + 1;
                    msg.what = MsgWhat.MSG_ANSW_RECO_ACTI_15;
                    m_handlerPostExecute.sendMessage(msg);

                }
            }, calendar.get(Calendar.YEAR), calendar.get(Calendar.MONTH), calendar.get(Calendar.DAY_OF_MONTH));
            break;
        }
        m_pickerDialog.setTitle(calendar.get(Calendar.YEAR) + " 年 " + (calendar.get(Calendar.MONTH) + 1)
                + " 月");
        return m_pickerDialog;
    }

Answer 1

 如果传入的id不等于0，m_pickerDialog.setTitle就报空指针了。