TypeErrror on Windows with some traitsui / mayavi tools

Hi,

let me link this from mayavi, I did not get any answer there and I feel this might be related: https://github.com/enthought/mayavi/issues/1127

In short, using mayavi works (examples, custom scripts) but when accessing the tree to change any property, the following occurs and crashes the current window:

```python
Traceback (most recent call last):
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\pyface\ui\qt4\action\action_item.py", line 184, in _qt4_on_triggered
self.controller.perform(action, action_event)
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\traitsui\qt4\tree_editor.py", line 1189, in perform
self.ui.do_undoable(self._perform, action)
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\traitsui\ui.py", line 655, in do_undoable
action(*args, **kw)
File "C:\Users\elm1rng\AppData\Local\Python\py-venv\bosch-def\lib\site-packages\traitsui\qt4\tree_editor.py", line 1192, in _perform
node, object, nid = self._data
TypeError: cannot unpack non-iterable NoneType object
```

Any help to pinpoint me for a fix? Seems to be that `self._data is None` which is not supposed to happen...?