which call gimp_item_get_by_id() and additionally check if the
returned item has the right type, and return NULL if not.

This is both shorter and more readable than

layer = GIMP_LAYER (gimp_item_get_by_id (id));

and additionally makes sure we don't cast e.g. a non-layer with
GIMP_LAYER(), which will give criticals but shouldn't, because the
wrong IDs can come from anywhere and are an input problem and not a
programming error (criticals are for programming errors).