Opening radiant, and then opening a .map file works. But opening it by doubleclicking it in RIKdev didn't.
Radiant
Windows 10 crash when opening a map file directly
Invalid • by Dinius • August 01, 2015 | edited Aug 01 2015
Can't reproduce on main PC (after """upgrading""" to windows 10 and getting drivers etc in order), so writing it off as one of a thousand bugs that MS will have to fix in windows 10.