H.-J. Heitländer <Heiner.Heitlaender@posteo.de> writes:

> 1) enter epub <C-x D> .../test.epub <ENTER>
>
> 2) switch to edit In DocView(?) <C-c C-c>
>
> 3) open file ...4.html <Enter>
>
> 4) change a line (for instance " " => " ")
>
> 5) save file <C-x C-f>
>
> 6) Symptom: minibuffer display "Wrong Type Argument: stringp, nil"

When this error happens, is the epub opened in archive-mode or
doc-view-mode?

When in doc-view-mode I get a different error:

| Debugger entered--Lisp error: (void-variable archive-nil-write-file-member)
|   (symbol-value archive-nil-write-file-member)
|   (archive-*-write-file-member archive archive-subfile-mode (symbol-value writer))
|   (if (fboundp writer) (funcall writer archive archive-subfile-mode) (archive-*-write-file-member archive archive-subfile-mode (symbol-value writer)))
|   (let ((writer (save-current-buffer (set-buffer archive-superior-buffer) (archive-name "write-file-member"))) (archive (save-current-buffer (set-buffer archive-superior-buffer) (archive-maybe-copy (buffer-file-name))))) (if (fboundp writer) (funcall writer archive archive-subfile-mode) (archive-*-write-file-member archive archive-subfile-mode (symbol-value writer))) (set-buffer-modified-p nil) (message "Updating archive...done"))
|   (save-restriction (message "Updating archive...") (widen) (let ((writer (save-current-buffer (set-buffer archive-superior-buffer) (archive-name "write-file-member"))) (archive (save-current-buffer (set-buffer archive-superior-buffer) (archive-maybe-copy (buffer-file-name))))) (if (fboundp writer) (funcall writer archive archive-subfile-mode) (archive-*-write-file-member archive archive-subfile-mode (symbol-value writer))) (set-buffer-modified-p nil) (message "Updating archive...done")) (set-buffer archive-superior-buffer) (if (not archive-remote) (revert-buffer) (archive-maybe-update nil)))
|   (save-excursion (save-restriction (message "Updating archive...") (widen) (let ((writer (save-current-buffer (set-buffer archive-superior-buffer) (archive-name "write-file-member"))) (archive (save-current-buffer (set-buffer archive-superior-buffer) (archive-maybe-copy (buffer-file-name))))) (if (fboundp writer) (funcall writer archive archive-subfile-mode) (archive-*-write-file-member archive archive-subfile-mode (symbol-value writer))) (set-buffer-modified-p nil) (message "Updating archive...done")) (set-buffer archive-superior-buffer) (if (not archive-remote) (revert-buffer) (archive-maybe-update nil))))
|   (archive-write-file-member)
|   (run-hook-with-args-until-success archive-write-file-member)
|   (basic-save-buffer nil)
|   (save-buffer)

When I hit C-c C-c to leave doc-view-mode and return to the archive view,
I get an error like you posted.

doc-view-mode doesn't remove its installed advice on
`buffer-revert-function' when hitting C-c C-c, and this simple fix seems
to help to avoid the error above (could you maybe try that please?):