Currently M-x guix-edit fails badly (actually ‘guix-package-location’)
pwhen passed the name of a nonexistent package:

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (error "Error in evaluating guile expression: ERROR: In procedure car:
ERROR: In procedure car: Wrong type (expecting pair): ()

Entering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.
scheme@(guile-user) [3]> ")
  signal(error ("Error in evaluating guile expression: ERROR: In procedure car:\nERROR: In procedure car: Wrong type (expecting pair): ()\n\nEntering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.\nscheme@(guile-user) [3]> "))
  error("Error in evaluating guile expression: %s" "ERROR: In procedure car:\nERROR: In procedure car: Wrong type (expecting pair): ()\n\nEntering a new prompt.  Type `,bt' for a backtrace or `,q' to continue.\nscheme@(guile-user) [3]> ")
  (if (geiser-eval--retort-error res) (error "Error in evaluating guile expression: %s" (geiser-eval--retort-output res)) (cdr (assq (quote result) res)))
  (let ((res (geiser-eval--send/wait (list (quote :eval) (list (quote :scm) str))))) (if (geiser-eval--retort-error res) (error "Error in evaluating guile expression: %s" (geiser-eval--retort-output res)) (cdr (assq (quote result) res))))
  (save-current-buffer (set-buffer (or repl (guix-geiser-repl))) (let ((res (geiser-eval--send/wait (list (quote :eval) (list (quote :scm) str))))) (if (geiser-eval--retort-error res) (error "Error in evaluating guile expression: %s" (geiser-eval--retort-output res)) (cdr (assq (quote result) res)))))
  guix-geiser-eval("(package-location-string \"foo\")" #<buffer *Guix Internal REPL*>)
  (car (guix-geiser-eval str repl))
  (replace-regexp-in-string "#t" "t" (car (guix-geiser-eval str repl)))
  (replace-regexp-in-string "#f\\|#<unspecified>" "nil" (replace-regexp-in-string "#t" "t" (car (guix-geiser-eval str repl))))
  (read (replace-regexp-in-string "#f\\|#<unspecified>" "nil" (replace-regexp-in-string "#t" "t" (car (guix-geiser-eval str repl)))))
  guix-geiser-eval-read("(package-location-string \"foo\")" #<buffer *Guix Internal REPL*>)
  guix-eval-read("(package-location-string \"foo\")")
  guix-package-location("foo")
  eval((guix-package-location "foo") nil)
  eval-expression((guix-package-location "foo") nil)
  call-interactively(eval-expression nil nil)
  command-execute(eval-expression)
--8<---------------cut here---------------end--------------->8---

I think this patch fixes it: