Crash on OSX 10.11/10.12 when changing fonts in Preferences

[mcdemarco]

This issue was initially reported in the Twine forums as happening on El Capitan.

I reproduced it on a (borrowed) mac running Sierra. A stack trace is attached:
stacktrace.txt

I could not reproduce it on Mavericks (10.9). I did not try Yosemite (10.10).

Steps to reproduce:

1. Open a fresh Twine 1.4.2 zip. (I just unzipped the one from twinery.org onto the desktop.)
2. Under the Twine menu, select Preferences.
3. Click on the first font shown to change it.

Twine crashes.

Unlike the initial report, I saw no other issues in the preferences (I could always open it, and change colors or check off checkboxes), and I got the font issue consistently.

This looks like a likely cause.