(void-function nil) error without backtrace

[Robert Widhopf-Fenk]

Hi,

I spend a considerable time to track down the following bug.

Whenever I enable debug-on-error when running VM I get a backtrace
with the following content:

,------------------------------------------
| Debugger entered--Lisp error: (void-function nil)
|  (nil)
`------------------------------------------

This is the WHOLE backtrace, well sometimes it is longer,
but never helpful!

See also the discussion in gnu.emacs.help (many thanks to Tim X) on
'How to debug "Debugger entered--Lisp error: (void-function nil)"'
at http://tinyurl.com/2uavle.

It was caused by a wrong toolbar item spec for the prop :button
and can be reproduced with GNU Emacs 22.0.92.1 and 21.4.1 on
Debian when adding the following to ~/.emacs:

(setq debug-on-error t)
(tool-bar-add-item "help" 'error 'buglet
                   ':button '(:toggle nil)
                   ':help "(void-function nil) buglet")

I am not sure how the backtrace should look like, but something like

,------------------------------------------
| Debugger entered--Lisp error: (void-function nil)
|  (nil)
| (tool-bar-redisplay)
| ...
`------------------------------------------

Would have saved me some days.

Robert.