title: python / urwid ListBox object not callable
body: <p>I've discovered urwid recently and started to play around with it. I've seen a Menu like tutorial example (which works!) and I've tried to modify the code a bit so I can get back and forth in the menu for starters - without doing anything else.</p>
  
  <p>However I got stuck - and Im at a loss on why its happening.</p>
  
  <p>Running this code:</p>
  
  <pre><code>    import urwid
  
      choices = u'Tank_1 Tank_2 Tank_3 Solarkreis Exit'.split()
  
      def menu(title, choices):
          body = [urwid.Text(title), urwid.Divider()]
          for c in choices:
              button = urwid.Button(c)
              if c == "Exit":
                  urwid.connect_signal(button, 'click', exit_program)
              else:
                  urwid.connect_signal(button, 'click', item_chosen, c)
              body.append(urwid.AttrMap(button, None, focus_map='reversed'))
  
  
          return urwid.ListBox(urwid.SimpleFocusListWalker(body))
  
      def item_chosen(button, choice):
          response = urwid.Text([u'Uebersicht ', choice, u'\n'])
          done = urwid.Button(u'Ok')
          urwid.connect_signal(done, 'click', menu(u'Menu', choices))
          main.original_widget = urwid.Filler(urwid.Pile([response,
              urwid.AttrMap(done, None, focus_map='reversed')]))
  
      def exit_program(button):
          raise urwid.ExitMainLoop()
  
      main = urwid.Padding(menu(u'Menu', choices), left=2, right=2)
      top = urwid.Overlay(main, urwid.SolidFill(u'\N{MEDIUM SHADE}'),
          align='center', width=('relative', 60),
          valign='middle', height=('relative', 60),
          min_width=20, min_height=9)
      urwid.MainLoop(top, palette=[('reversed', 'standout', '')]).run()
  </code></pre>
  
  <p>Running it I get</p>
  
  <pre><code>TypeError: 'ListBox' object is not callable
  </code></pre>
  
  <p>And I done quite understand how and why.. Help is much appreciated!</p>
  
  <p>Edit: Forgot to add, the error pops up when I try to go back from the sub - menu to the original menu.</p>