Crash when "Files" is not enabled in Gnome Online Accounts
When you add your NextCloud/OwnCloud server in Gnome, you have he option to enable Calendar, Contact, Documents, and Files.
I didn't have "Files" enable.
When I created a note and pressed that "back" button in Gnome Notes, it crashed.
Then, I enable Files in Gnome Online Accounts.
When I tried it again, it synced properly.