Copied text is lost after dialog is closed

To reproduce (at least with qt backend):

1. Run the code below
2. Copy the text to clipboard
3. Close the traits dialog
4. Try to paste
5. Be sad

```python
from traits.api import HasTraits, Str
from traitsui.api import Item, View

class Foo(HasTraits):
x = Str('why hello there')
traits_view = View(Item('x'))

if __name__ == '__main__':
Foo().configure_traits()
```