;;; SOLVED!
;;;
;;; Anyone knows why I'm getting:
;;;
;;;     Wrong type argument: char-or-string-p, nil
;;;
;;; after running `detubifier-detubify-region' on a region like:
;;;
;;; [mark]youtube.com[point]
;;;
;;; ?

(defcustom detubifier-regexp-replacement-pairs
  '(("www\\.youtube\\.com" . "invidious.jing.rocks")
    ("youtube\\.com" . "invidious.jing.rocks")
    ("youtu\\.be/" . "invidious.jing.rocks/watch?v=")
    ("twitter.com" . "nitter.poast.org"))
  "List of pairs, each with a regexp and its replacement."
  :type '(list (cons regexp string))
  :group 'detubifier)

(defun detubifier-detubify-region ()
  (interactive)
  (save-excursion
   (replace-region-contents (min (point) (mark))
                            (max (point) (mark))
                            (lambda ()
                              (dolist (regexp-replacement-pairs detubifier-regexp-replacement-pairs)
                                (pcase regexp-replacement-pairs
                                  (`(,current-regexp . ,current-replacement)
                                   (save-excursion
                                     (while (re-search-forward current-regexp nil t)
                                       (replace-match current-replacement nil nil))))))))))

;;; Turns out `replace-region-contents' needs a buffer or a string as
;;; the return value of the provided thunk with which to replace the
;;; marked region.
;;;
;;; This works better:

(defun detubifier-detubify-region ()
  "Replace, in region, all occurances of the regular expressions in `detubifier-regexp-replacement-pairs' with their replacement targets."
  
  (interactive)

  (save-excursion
    (save-restriction ;; Save narrowing setting and revert to it when finishing.
      (narrow-to-region (mark) (point))
      (dolist (regexp-replacement-pairs detubifier-regexp-replacement-pairs)
        (pcase regexp-replacement-pairs
          (`(,current-regexp . ,current-replacement)
           (goto-char (point-min))
           (while (re-search-forward current-regexp nil t)
             (replace-match current-replacement nil nil))))))))