Please the the UDN for more details.
The crash happens because the map is dirtied on load, and then later attempted to be unloaded, which fails. The call that dirties the map on load has been introduced in CL 26465295.
I've confirmed that reverting that CL prevents the crash.
I've attached callstacks for the crash itself, the modal popup that claims the map could not be unloaded and the callstack that hits the code that dirties the map's package.
It's unclear why the editor tries to unload the map in the first place (a different map was open before, so it should not be necessary to unload it at all, only load it once).
See the linked UDN for detailed repro steps.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-205613 in the post.