NULL is OK, both for passing and for returning.
This parameter is a 'user_data', for callbacks; many bindings can pass NULL here.
Generics and defining elements of containers and arrays.
The callback is valid only during the call to the method.
The intention of a Stable interface is to enable arbitrary third parties to develop applications to these interfaces, release them, and have confidence that they will run on all minor releases of the product (after the one in which the interface was introduced, and within the same major release). Even at a major release, incompatible changes are expected to be rare, and to have strong justifications.
Free data after the code is done.