I think I found the problem that cause the reboot. It is the debug print out.
If I comment out this line:
lv_log_register_print_cb( my_print ); /* register print function for debugging */
It works as normal. Not sure how the lv_log_register_print_cb is causing the reboot.
Another thing, the LV_DEMO_WIDGETS_SLIDESHOW is not working. No slide demo.