I can't help with the particular issue, but from a diagnostic standpoint, I would try to edit it with another editor, to eliminate N++ as a point of contention. I'd also try editing the text directly in the resource via the OBS interface (rather than telling it to use a file). Not permanently; just to see if the problem can be reproduced in that way.
Also, your OBS log may contain some helpful information just before that happened (and when OBS crashed, it may have indicated that it will save a crash report...?)
It would also be helpful to know what OS/version you're using.