Android Null Pointer的有关问题

我在主activity里面设置了有三个tab的切换卡，在第三个tab里面是个ListView，单击ListView会弹出时间选择对话框public void onSetWarn(){        LayoutInflater factoryInflater = LayoutInflater.from(MainActivity.this);        final View dialogView = factoryInflater.inflate(R.layout.dialog, null);        AlertDialog dialog = new AlertDialog.Builder(MainActivity.this).setTitle("设置")                .setView(dialogView)                .setPositiveButton("确定", new DialogInterface.OnClickListener() {                                        public void onClick(DialogInterface dialog, int which) {                        Toast.makeText(getApplicationContext(), "设置成功！", Toast.LENGTH_SHORT).show();                    }                })                .setNegativeButton("取消", new DialogInterface.OnClickListener() {                                        public void onClick(DialogInterface dialog, int which) {                                            }                }).create();        /*        timePicker = (TimePicker) this.findViewById(R.id.TimePicker1);        timePicker.setIs24HourView(true);        */        dialog.show();    }