bug#13263: 24.2.91; browse-url: (wrong-type-argument stringp nil)

[Leo]

While mouse-clicking on links in HTML articles in gnus (using shr), I
sometimes get this error:

Debugger entered--Lisp error: (wrong-type-argument stringp nil)
  string-match("\\`mailto:"; nil)
  browse-url(nil)
  widget-url-link-action((url-link :args nil :value nil :delete 
widget-leave-text :help-echo nil :keymap (keymap (13 . shr-browse-url) (111 . 
shr-save-contents) (118 . shr-browse-url) (117 . shr-copy-url) (73 . 
shr-insert-image) (122 . shr-zoom-image) (105 . shr-browse-image) (97 . 
shr-show-alt-text)) :from #<marker (moves after insertion) at 2625 in *Article 
gwene.org.python.planet*> :to #<marker at 2644 in *Article 
gwene.org.python.planet*> :button-overlay #<overlay from 2625 to 2644 in 
*Article gwene.org.python.planet*>) (mouse-1 (#<window 908 on *Article 
gwene.org.python.planet*> 2639 (132 . 529) 1728833761 nil 2639 (14 . 29) nil (6 
. 7) (9 . 18))))
  widget-apply((url-link :args nil :value nil :delete widget-leave-text 
:help-echo nil :keymap (keymap (13 . shr-browse-url) (111 . shr-save-contents) 
(118 . shr-browse-url) (117 . shr-copy-url) (73 . shr-insert-image) (122 . 
shr-zoom-image) (105 . shr-browse-image) (97 . shr-show-alt-text)) :from 
#<marker (moves after insertion) at 2625 in *Article gwene.org.python.planet*> 
:to #<marker at 2644 in *Article gwene.org.python.planet*> :button-overlay 
#<overlay from 2625 to 2644 in *Article gwene.org.python.planet*>) :action 
(mouse-1 (#<window 908 on *Article gwene.org.python.planet*> 2639 (132 . 529) 
1728833761 nil 2639 (14 . 29) nil (6 . 7) (9 . 18))))
  widget-apply-action((url-link :args nil :value nil :delete widget-leave-text 
:help-echo nil :keymap (keymap (13 . shr-browse-url) (111 . shr-save-contents) 
(118 . shr-browse-url) (117 . shr-copy-url) (73 . shr-insert-image) (122 . 
shr-zoom-image) (105 . shr-browse-image) (97 . shr-show-alt-text)) :from 
#<marker (moves after insertion) at 2625 in *Article gwene.org.python.planet*> 
:to #<marker at 2644 in *Article gwene.org.python.planet*> :button-overlay 
#<overlay from 2625 to 2644 in *Article gwene.org.python.planet*>) (mouse-1 
(#<window 908 on *Article gwene.org.python.planet*> 2639 (132 . 529) 1728833761 
nil 2639 (14 . 29) nil (6 . 7) (9 . 18))))
  ......

The raw article can be found here: http://bpaste.net/raw/q1PRRp7kYfHbfaSLQB0x/

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