Traceback when cancelling open volume file/chaco file

To reproduce:
- start mayavi2, select from the menubar `File->Load data->Volume file`
- Click cancel on the resulting dialog.

```
Traceback (most recent call last):
File "/Users/sborini/.edm/envs/simphony/lib/python2.7/site-packages/pyface-5.1.0-py2.7.egg/pyface/ui/qt4/action/action_item.py", line 163, in _qt4_on_triggered
self.controller.perform(action, action_event)
File "/Users/sborini/.edm/envs/simphony/lib/python2.7/site-packages/pyface-5.1.0-py2.7.egg/pyface/workbench/action/action_controller.py", line 35, in perform
return action.perform(event)
File "/Users/sborini/Work/github/enthought/mayavi/mayavi/action/sources.py", line 101, in perform
obj = callable()
File "/Users/sborini/Work/github/enthought/mayavi/mayavi/sources/volume_reader.py", line 53, in __init__
self.file_prefix = self.reader.file_prefix
File "/Users/sborini/.edm/envs/simphony/lib/python2.7/site-packages/traits-4.5.0-py2.7-macosx-10.6-x86_64.egg/traits/trait_handlers.py", line 172, in error
value )
traits.trait_errors.TraitError: The 'file_prefix' trait of a VolumeReader instance must be a string, but a value of None was specified.
```

The same behavior is present with the chaco file option

```
Traceback (most recent call last):
File "/Users/sborini/.edm/envs/simphony/lib/python2.7/site-packages/pyface-5.1.0-py2.7.egg/pyface/ui/qt4/action/action_item.py", line 163, in _qt4_on_triggered
self.controller.perform(action, action_event)
File "/Users/sborini/.edm/envs/simphony/lib/python2.7/site-packages/pyface-5.1.0-py2.7.egg/pyface/workbench/action/action_controller.py", line 35, in perform
return action.perform(event)
File "/Users/sborini/Work/github/enthought/mayavi/mayavi/action/sources.py", line 101, in perform
obj = callable()
File "/Users/sborini/Work/github/enthought/mayavi/mayavi/sources/chaco_reader.py", line 51, in __init__
self.base_name = self.reader.base_name
File "/Users/sborini/.edm/envs/simphony/lib/python2.7/site-packages/traits-4.5.0-py2.7-macosx-10.6-x86_64.egg/traits/trait_handlers.py", line 172, in error
value )
traits.trait_errors.TraitError: The 'base_name' trait of a ChacoReader instance must be a string, but a value of None was specified.
```