Adding a new node via QT TreeEditor fails under specific circumstances

When trying to add a new element to an empty TreeNode with auto_open = False the application will crash with the following Python error message
`Traceback (most recent call last):
File "C:\Apps\Continuum\anaconda2\envs\py37AutoSum\lib\site-packages\traitsui\qt4\tree_editor.py", line 1042, in _on_tree_sel_changed
expanded, node, sel_object = self._get_node_data(nid)
File "C:\Apps\Continuum\anaconda2\envs\py37AutoSum\lib\site-packages\traitsui\qt4\tree_editor.py", line 893, in _get_node_data
return nid._py_data
AttributeError: 'QTreeWidgetItem' object has no attribute '_py_data'`

**Steps to reproduce:**
- Environment: Python 3.7.3, qt 5.9.7, TraitsUI 6.1.3 and Traits 5.1.2
- Run the file attached below which is based on the `tree_editor.py` example.
- Right click on _Marketing_ and select _New_ and click on _Employee_. The application will then crash with the error message shown above

[InsertNodeTreeEditorBug.txt](https://github.com/enthought/traitsui/files/3665862/InsertNodeTreeEditorBug.txt)