Hello, I use org-refile quite a bit, but for my use-case, I want to refile
my org-mode subtrees not to an org-agenda file, but to the current file or
another file I'm currently visiting.

I had a way of doing this that was working correctly, but now has stopped
working. I'm not sure what I changed that broke it.

Here's the code I'm using for org-refile:

(setq org-outline-path-complete-in-steps nil) ; Refile in a single go
(setq org-completion-use-ido nil)
(setq org-refile-use-outline-path t) ; Show full paths for refiling
(setq org-refile-allow-creating-parent-nodes (quote confirm)) ; allow
refile to create parent tasks with confirmation

(defun my-org-files-list ()
 (mapcar (lambda (buffer)
   (buffer-file-name buffer))
   (org-buffer-list 'files t)))

(setq org-refile-targets '((my-org-files-list :maxlevel . 3)))

Again, this used to work. But now when I run org-refile, I get this:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  get-file-buffer(nil)
  org-find-base-buffer-visiting(nil)
  org-get-agenda-file-buffer(nil)
  org-refile-get-targets(nil (#("this" 0 4 (fontified t face org-level-1))))
  org-refile-get-location("Refile subtree \"this\" to" nil confirm nil)
  org-refile(nil)
  call-interactively(org-refile record nil)
  command-execute(org-refile record)
  execute-extended-command(nil "org-refile")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)

Any suggestions on how to fix this? Thanks!