tutor.py freezes when expanding child items

I'm failing to get the `tutor.py` script to work with Traits master and the latest EDM-available releases for other packages.

Steps to reproduce:
```
$ python etstool.py install
$ edm run -e traits-test-3.6 -- python examples/tutorials/tutor.py
```
At this point the tutor application launches successfully. But if I then click on the displayed "Traits" child item in the tree view, the application freezes, I get the macOS spinning rainbow, CPU goes to 100%, and I have to do a Force Quit.

Packages and versions:
```
mirzakhani:traits mdickinson$ edm list -e traits-test-3.6
# Packages in environment 'traits-test-3.6'
# prefix: '/Users/mdickinson/.edm/envs/traits-test-3.6'
alabaster 0.7.10-1 enthought/free
appdirs 1.4.3-1 enthought/free
babel 2.6.0-1 enthought/free
certifi 2018.11.29-1 enthought/free
chardet 3.0.4-1 enthought/free
configobj 5.0.6-3 enthought/free
configparser 3.5.0-1 enthought/free
coverage 4.3.4-1 enthought/free
cython 0.29.13-1 enthought/free
distribute_remove 1.0.0-4 enthought/free
docutils 0.16-2 enthought/free
enthought_sphinx_theme 0.6.2-1 enthought/free
entrypoints 0.3.0-2 enthought/free
flake8 3.7.7-1 enthought/free
gnureadline 6.3.8-1 enthought/gpl
idna 2.5-1 enthought/free
imagesize 0.7.1-1 enthought/free
jinja2 2.10.1-1 enthought/free
markupsafe 1.1.1-1 enthought/free
mccabe 0.6.1-1 enthought/free
mkl 2018.0.3-2 enthought/free
mypy 0.761-1 enthought/free
mypy_extensions 0.4.3-1 enthought/free
numpy 1.17.4-1 enthought/free
packaging 16.8-3 enthought/free
pip 20.0.2-1 enthought/free
pycodestyle 2.5.0-1 enthought/free
pyface 7.0.0-1 enthought/free
pyflakes 2.1.1-1 enthought/free
pygments 2.2.0-1 enthought/free
pyparsing 2.2.0-1 enthought/free
pyqt 4.12.1-1 enthought/free
python_version 0.0.2-1 enthought/free
pytz 2018.9-1 enthought/free
qt 4.8.7-12 enthought/free
requests 2.21.0-3 enthought/free
setuptools 41.6.0-3 enthought/free
sip 4.19.2-2 enthought/free
six 1.11.0-1 enthought/free
snowballstemmer 1.2.1-1 enthought/free
sphinx 2.3.1-2 enthought/free
sphinxcontrib_applehelp 1.0.1-2 enthought/free
sphinxcontrib_devhelp 1.0.1-2 enthought/free
sphinxcontrib_htmlhelp 1.0.2-2 enthought/free
sphinxcontrib_jsmath 1.0.1-2 enthought/free
sphinxcontrib_qthelp 1.0.2-2 enthought/free
sphinxcontrib_serializinghtml 1.1.3-2 enthought/free
sphinxcontrib_websupport 1.1.0-1 enthought/free
traitsui 7.0.0-1 enthought/free
typed_ast 1.4.1-1 enthought/free
typing 3.7.4.1-1 enthought/free
typing_extensions 3.7.4.1-1 enthought/free
urllib3 1.25.3-2 enthought/free
```