I'm willing to bet this has something to do with something plugging into system in an odd way, or a bad parameter being passed into a kernel level call by a library. I don't have my device setup for debugging, but if someone who can get the SaveAs dialog to work in v48 could do an strace and find out what calls are being done, I'd bet good money something's calling in to the same system call in v48 (probably something around getting mount points) with a sometimes bad parameter.

User-based apps shouldn't be able to crash a system this readily. There's got to be something happening at a kernel/system level to cause this quick of a reboot on multiple people's systems.