In QEMU 4.2.0, a MemoryRegionOps object may lack read/write callback methods, leading to a NULL pointer dereference.
https://lists.gnu.org/archive/html/qemu-devel/2020-06/msg09961.html
https://lists.debian.org/debian-lts-announce/2022/09/msg00008.html
https://lists.debian.org/debian-lts-announce/2021/02/msg00024.html