nil has a plist, of course.

[Stephen J. Turnbull]

Aidan Kehoe writes:

 > And on two non-vanilla invocations of mine it’s non-nil. These seem like
 > bugs:

Easy to reproduce:

(with-temp-buffer            ;; let's not be visting a file
  (defcustom junk nil "junk custom")
  (assq 'junk (cadr (object-plist nil))))
=> (junk custom-variable)

Easy to prevent:

(with-temp-buffer
  (defgroup trash-group nil "just for laughs")
  (defcustom trash nil "trash custom")
  (cons (assq 'trash (cadr (object-plist nil)))
        (assq 'trash (cadr (object-plist 'trash-group)))))
=> (nil trash custom-variable)

Question: Should we *require* that customizations be in a group (ie,
defcustom barfs if there's no :group argument and (custom-current-group)
is nil)?  Or should we force `custom-current-group' to return something
global like 'xemacs rather than nil?  Or should we allow anti-social
customizations, that avoid all groups?

I think option 2 is probably safest and least invasive, although the
pedant in me hankers after option 1.  Option 3 strikes me as rather
risky given the general fragility of customize.