Invoke WindowPalette.popdown when menu property is None

[alsroot]

PPA:  https://launchpad.net/~alsroot/+archive/sugar-0.86

Can't find 100% way to reproduce this bug.

Traceback (most recent call last):
  File "/usr/lib/pymodules/python2.6/gtk-2.0/gobject/__init__.py", line 101, in obj_set_property
    prop.setter(self, value)
  File "/usr/lib/pymodules/python2.6/sugar/graphics/palettewindow.py", line 645, in set_palette
    self._palette.popdown(immediate=True)
  File "/usr/lib/pymodules/python2.6/sugar/graphics/palette.py", line 204, in popdown
    self._popdown_submenus()
  File "/usr/lib/pymodules/python2.6/sugar/graphics/palette.py", line 212, in _popdown_submenus
    for menu_item in self.menu.get_children():
AttributeError: 'NoneType' object has no attribute 'get_children'

[alsroot]

dup of #1338

[alsroot]

 http://git.sugarlabs.org/projects/sugar-toolkit/repos/mainline/commits/ecee594190f7534598b4e7f0233d81525099ab7f