CompoundEditor_demo.py segfault on qt

It is possible this is the expected behavior but I was confused.

- [ ] When using Qt, if you click for example 'C' in the custom section, and then in the text section try to input an integer error messages pop up and when you close them it seg faults.

```
(qt-env)~/Desktop/traitsui/traitsui/examples/demo/Standard_Editors aayres (fix-CompoundEditor_demo) $ python CompoundEditor_demo.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
```

- [x] On wx however, you simply get the pop ups giving a value error, but the GUI doesn't close and there is no segfault. Once you manual close it, there is an error in the command line namely

```
$ python CompoundEditor_demo.py
03:35:06 PM: Debug: Adding duplicate image handler for 'Windows bitmap file'
03:35:06 PM: Debug: Adding duplicate animation handler for '1' type
03:35:06 PM: Debug: Adding duplicate animation handler for '2' type
03:35:06 PM: Debug: Adding duplicate image handler for 'Windows bitmap file'
03:35:06 PM: Debug: Adding duplicate animation handler for '1' type
03:35:06 PM: Debug: Adding duplicate animation handler for '2' type
03:35:06 PM: Debug: Adding duplicate image handler for 'Windows bitmap file'
03:35:06 PM: Debug: Adding duplicate animation handler for '1' type
03:35:06 PM: Debug: Adding duplicate animation handler for '2' type
Traceback (most recent call last):
File "/Users/aayres/Desktop/traitsui/traitsui/wx/editor_factory.py", line 125, in update_object
self.value = self.control.GetValue()
AttributeError: 'NoneType' object has no attribute 'GetValue'
```

It seems this error or one very similar has been mentioned in #623