File(exists=True) trait disables OK button permanently

Example code:
```python
from traits.api import File, HasTraits

class DummyModel(HasTraits):
file_path = File(exists=True)

model = DummyModel()
model.configure_traits()
```

If one switch the focus to another window and then come back to the `DummyModel` window, the `OK` button will be disabled permanently. It won't be re-enabled even one choose a valid file later.