Hello Adria,
You're right, by default, if you use the configuration without defining a callback this will compare it with the NULL_CALLBACK and will crash, the best way to change this will be to replaced in the configuration the callback name for the NULL_CALLBACK. If you do this and do more changes in the peripheral view this name will keep it.

Also is important that you don't modify the drivers and the files in the SDK folder since these are managed by the config tools and any changes done will be replaced. If you want to create any API related to the driver I will suggest to create your own .c and .h
Best Regards,
Alexis Andalon