NULL is OK, both for passing and for returning.
This symbol is a constructor, not a static method.
NULL may be passed as the value in, out, in-out; or as a return value.
Parameter for returning results. Default is transfer full.
The callback is valid only during the call to the method.
The callback is valid until the GDestroyNotify argument is called.
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.
Don't free data after the code is done.