Type checking for `define-widget'

* lisp/widget.el (define-widget): Check that documentation is a string
or nil; prevent wailing and gnashing of teeth when users forget to
pass a docstring and wonder why their properties don't work.

lisp/ChangeLog

@@ -1,5 +1,9 @@
 2015-03-14  Daniel Colascione  <dancol@dancol.org>
 
+	* widget.el (define-widget): Check that documentation is a string
+	or nil; prevent wailing and gnashing of teeth when users forget to
+	pass a docstring and wonder why their properties don't work.
+
 	* startup.el (command-line): Process "--no-x-resources".
 
 2015-03-13  Kevin Ryde  <user42_kevin@yahoo.com.au>

lisp/widget.el

@@ -83,6 +83,9 @@ create identical widgets:
 * (apply 'widget-create CLASS ARGS)
 
 The third argument DOC is a documentation string for the widget."
+  ;;
+  (unless (or (null doc) (stringp doc))
+    (error "widget documentation must be `nil' or a string."))
   (put name 'widget-type (cons class args))
   (put name 'widget-documentation (purecopy doc))
   name)