Hello! I'm new user. Just read the Manual. And I'd like to report a bug 
or ask a question (if it's not a bug). So, as it said in the Manual, the 
TUTORIAL file that I read is just a copy of the original, so I can 
change it freely. But. When I change it, and then triing to leave Emacs 
(Ctrl + x + c), I got this message.

Error on exit:
tutorial--saved-file: Wrong type argument: stringp, nil

The debugger says:
Debugger entered--Lisp error: (wrong-type-argument stringp nil)
   file-name-extension(nil)
   tutorial--saved-file()
   tutorial--save-on-kill()
   run-hook-with-args-until-failure(tutorial--save-on-kill)
   save-buffers-kill-emacs(nil)
   save-buffers-kill-terminal(nil)
   funcall-interactively(save-buffers-kill-terminal nil)
   command-execute(save-buffers-kill-terminal)

The error appears only when you try to exit and the TUTORIAL file is 
modified. At the same time, Emacs allows saving the TUTORIAL file by 
Ctrl + x + s during operation.

Regards, Tony.