Hi,

According to the documentation, the variable ido-completion-buffer may
be set to nil to prevent completion buffers from popping up.  However,
if this variable is set to nil, it becomes impossible to switch buffers
or visit files.

Recipe:

1. emacs -Q

2. enable ido
(ido-mode t)
(setq ido-completion-buffer nil)

3. Try to switch buffers or visit a file
C-x b RET (or C-x C-f RET)
The error "Wrong type argument: stringp, nil" flashes in the minibuffer
and nothing happens.

I have included a patch to lisp/ido.el that should fix the problem by
preventing (get-buffer nil) from being called.

Regards,
Anmol

In GNU Emacs 24.0.94.5 (x86_64-apple-darwin11.3.0, NS apple-appkit-1138.32)
of 2012-03-12 on morpheus.lan
Windowing system distributor `Apple', version 10.3.1138
Configured using:
`configure '--with-ns''

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t