As found in 6f2b39c1be6a8b9531264d92138dc2629d51cfa0, you can't just register a control callback by itself, you can only register them in the set config callback, (or, at least, _after_ set config has been done. This has been compile tested only for the extra examples that were found to have this failing pattern.
README
This is small USB controlled LED blinking example program using libopencm3.
It's intended for the ST STM32-based Olimex STM32-H107 eval board. The usbtest.py script in this directory maybe used to control the LED.