The exception nuisance mentioned at the bottom of page 155 causes an exception and hangs the program. We don’t know how to deal with exceptions elegantly yet, but …
Near the bottom of RMDocument.m is a function
Changing YES to NO seems to fix it for now.
ps When you get to chapter 10, you will find the choices in the File menu vary according to whether autosavesInPlace returns YES or NO. Why?