Settable_cached_property.py Seg fault

https://github.com/enthought/traitsui/blob/master/traitsui/examples/demo/Advanced/Settable_cached_property.py

the docstring instructs a user to "Now try typing a value into the 'd' trait and you will see that an error
results (indicated by the text entry field turning red), because this is a
normal 'cached' trait that has no 'setter' method defined."

When I do this on the lower text field for D and try to change the text field becomes red as expected. However, on the text field on the right hand side if I change D and hit enter, 2 pop up windows appear saying "The 'd' trait of a SettableCachedProperty instance is 'read only'." Then, when I close those windows the GUi closes and I see

```
$ python Settable_cached_property.py
Segmentation fault: 11
```
In the command line.

I believe this actually may be the same issue as #1073 .

```
(qt-env)~/Desktop/traitsui/traitsui/examples/demo/Advanced aayres (fix-Settable_cached_property) $ python Settable_cached_property.py
Qt has caught an exception thrown from an event handler. Throwing
exceptions from an event handler is not supported in Qt.
You must not let any exception whatsoever propagate through Qt code.
If that is not possible, in Qt 5 you must at least reimplement
QCoreApplication::notify() and catch all exceptions there.

Segmentation fault: 11
```