Well, it's not a big problem though. But I can describe my actions as an algorithm for you to make it easier to reproduce the bug:

1. I start Emacs, it starts with my minor custom changes that I added to .emacs file.

2. Open an Emacs Tutorial with a mouse click and answer Yes to load my remembered position.

3. Put some changes into the Tutorial. Say, just add a new line.

4. Press C-x-s to save. Emacs asks me to write a filename, I write TUTORIAL, because the full path is alredy shown. Then Emacs asks me whether I want to overwrite the file. I anser Yes (type y).

5. Minibuffer shows me message "Wrote c:\...path...TUTORIAL"

6. I Press C-x-c to exit. It shows "Save your position in the tutorial?". Answer Yes (y).

7. Minibuffer shows "Wrong type argument: stringp, nil". And doesn't quit!

8. Then I press C-x-c again, but now I choose No (n). And Emacs exits. Tutorial file is not saved (I can see it by opening it again and check)!

So, here is how it is going on. But note, that if I skip the 4-th item (don't press C-x-s to save) and press C-x-c right after changing the Tutorial (addding a new line), Emacs quits normally and the Tutorial file is being saved! So, I guess it is a C-x-s function that triggers problem.

Hope it will help you cathing the bug.

Tatsu Takamaro <tatsu.takamaro@gmail.com> writes:

In GNU Emacs 29.4 (build 2, x86_64-w64-mingw32) of 2024-07-05 built on
 AVALON
Windowing system distributor 'Microsoft Corp.', version 10.0.19045
System Description: Microsoft Windows 10 Pro (v10.0.2009.19045.5131)

I just tried to reproduce it with Emacs 29.4 (not on windows though) and
I was not able to get the same error as you.

What is strange is that there is only one call to `file-name-extension'
in `tutorial--saved-file' and at this point, as you've seen,
`tutorial--lang' is set to something meaningful (not nil).