Transient error in modal dialog tests

The following transient failure occurred on a Travis CI cron run (Python 2.7, PyQt4):
```
======================================================================
ERROR: test_parent (pyface.tests.test_confirmation_dialog.TestConfirmationDialog)
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Traceback (most recent call last):
File "/home/travis/.edm/envs/pyface-test-2.7-pyqt/lib/python2.7/site-packages/pyface-6.1.0.dev38-py2.7.egg/pyface/tests/test_confirmation_dialog.py", line 270, in test_parent
tester.open_and_run(when_opened=lambda x: x.close(accept=True))
File "/home/travis/.edm/envs/pyface-test-2.7-pyqt/lib/python2.7/site-packages/pyface-6.1.0.dev38-py2.7.egg/pyface/ui/qt4/util/modal_dialog_tester.py", line 141, in open_and_run
self.assert_no_errors_collected()
File "/home/travis/.edm/envs/pyface-test-2.7-pyqt/lib/python2.7/site-packages/pyface-6.1.0.dev38-py2.7.egg/pyface/ui/qt4/util/modal_dialog_tester.py", line 267, in assert_no_errors_collected
raise type_(msg.format('\n\n'.join(tracebacks)))
AttributeError: The following error(s) were detected:
Traceback (most recent call last):
File "/home/travis/.edm/envs/pyface-test-2.7-pyqt/lib/python2.7/site-packages/pyface-6.1.0.dev38-py2.7.egg/pyface/ui/qt4/util/modal_dialog_tester.py", line 249, in capture_error
yield
File "/home/travis/.edm/envs/pyface-test-2.7-pyqt/lib/python2.7/site-packages/pyface-6.1.0.dev38-py2.7.egg/pyface/ui/qt4/util/modal_dialog_tester.py", line 233, in close
self._gui.invoke_later(widget.accept)
AttributeError: 'NoneType' object has no attribute 'accept'
----------------------------------------------------------------------
```
This smells to me of a race condition, but I could be wrong.