Window close tests fails on BadWindow errors

Here's an example of test that might fail because of BadWindow errors at AP bamf level:

ERROR: unity.tests.test_panel.PanelWindowButtonsActionsTests.test_window_buttons_close_follows_fitts_law(Single Monitor,Global Menus)
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Empty attachments:
  /var/log/syslog
  unity-log

Traceback (most recent call last):
  File "/home/marco/unity/tests/autopilot/unity/tests/test_panel.py", line 672, in test_window_buttons_close_follows_fitts_law
    self.assertNoWindowOpenWithXid(win_xid)
  File "/home/marco/unity/tests/autopilot/unity/tests/test_panel.py", line 159, in assertNoWindowOpenWithXid
    self.assertThat(refresh_fn, Eventually(Equals([])))
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 431, in assertThat
    mismatch_error = self._matchHelper(matchee, matcher, message, verbose)
  File "/usr/lib/python2.7/dist-packages/testtools/testcase.py", line 481, in _matchHelper
    mismatch = matcher.match(matchee)
  File "/usr/lib/python2.7/dist-packages/autopilot/matchers/__init__.py", line 113, in match
    wait_fun(self.matcher, self.timeout)
  File "/usr/lib/python2.7/dist-packages/autopilot/matchers/__init__.py", line 132, in _callable_wait_for
    failure_msg = mismatch.describe()
  File "/usr/lib/python2.7/dist-packages/testtools/matchers/_basic.py", line 76, in describe
    right = repr(self.other)
  File "/usr/lib/python2.7/dist-packages/autopilot/process/_bamf.py", line 596, in __repr__
    self.title if self._x_win else '', self.x_id)
  File "/usr/lib/python2.7/dist-packages/autopilot/process/_bamf.py", line 487, in title
    return self._getProperty('_NET_WM_NAME')
  File "/usr/lib/python2.7/dist-packages/autopilot/process/_bamf.py", line 606, in _getProperty
    get_display().get_atom(_type), X.AnyPropertyType)
  File "/usr/lib/python2.7/dist-packages/Xlib/xobject/drawable.py", line 452, in get_full_property
    prop = self.get_property(property, type, 0, sizehint)
  File "/usr/lib/python2.7/dist-packages/Xlib/xobject/drawable.py", line 441, in get_property
    long_length = length)
  File "/usr/lib/python2.7/dist-packages/Xlib/protocol/rq.py", line 1478, in __init__
    self.reply()
  File "/usr/lib/python2.7/dist-packages/Xlib/protocol/rq.py", line 1498, in reply
    raise self._error
BadWindow: <class 'Xlib.error.BadWindow'>: code = 3, resource_id = Xlib.xobject.resource.Resource(0x04a00085), sequence_number = 4570, major_opcode = 20, minor_opcode = 0