WxPython font dialog crashes after cancel

## Environment

OS: _all_
Python version: _3.6_
Toolkit:_Wx_

## Description

When using a FontDialog under Wx, when you cancel you get a crash with a traceback like this:
```
Traceback (most recent call last):
File "font_dialog.py", line 14, in
font = get_font(None, font='12 pt Helvetica sans-serif')
File "/Users/cwebster/src/process/pyface/pyface/font_dialog.py", line 37, in get_font
result = dialog.open()
File "/Users/cwebster/src/process/pyface/pyface/i_dialog.py", line 168, in open
self.close()
File "/Users/cwebster/src/process/pyface/pyface/ui/wx/font_dialog.py", line 51, in close
self.font = Font.from_toolkit(wx_font)
File "/Users/cwebster/src/process/pyface/pyface/font.py", line 354, in from_toolkit
return cls(**toolkit_font_to_properties(toolkit_font))
File "/Users/cwebster/src/process/pyface/pyface/ui/wx/font.py", line 160, in toolkit_font_to_properties
family = wx_family_to_generic_family[toolkit_font.GetFamily()]
wx._core.wxAssertionError: C++ assertion "IsOk()" failed at /Users/robind/projects/bb2/dist-osx-py36/build/ext/wxWidgets/src/common/fontcmn.cpp(407) in GetFamily(): invalid font
```

## Steps to Reproduce

Install a Wx environment, run examples/dialogs/font_dialog.py in it, and cancel the dialog when it is done.