Notes app broke on Mac Pro after macOS Sierra upgrade
I'm not sure currently, but basically some text/special symbols is to blame.
As stated here: http://forums.macrumors.com/threads/notes-crashing-in-macos-sierra.2000203/
Katalyst2 said: ↑ Me too -- mid-2013 MBA 13. Damn annoying!
EDIT: And as usual, as soon as I post a problem report, I somehow manage to fix it :)
What I just did now was log into iCloud.com and I made a tiny edit to one of my notes (in my case, I deleted one line on a to-do list). Then I waited a few seconds for the data to refresh, and then I tried opening Notes locally again.
Let us know if this works.