Edit window fix

Background
-----
This pull request fixes a crash issue with node editing on ```Revit``` (see below for detailed test scenarios). The crash originated from the following statement on ```Revit```:

```cs
public EditWindow(bool updateSourceOnTextChange = false)
{
    // In unmanaged host scenarios like Revit, 'Application.Current' will be 'null'.
    this.Owner = WPF.FindUpVisualTree<DynamoView>(Application.Current.MainWindow);
}
```

Test Scenarios
-----
The fix has been tested on the following scenarios:

1. Click ```Edit...``` on context menu of a ```Note```
2. Click ```Rename Node...``` on context menu of a ```Node```
3. Click ```Edit...``` on context menu of a ```String``` node
4. Click ```Edit...``` on ```Volume From String``` node

Notes for Reviewers
-----
I have not been able to hit ```editWindowItem_Click``` method in ```String``` class (in ```BasicInteractive.cs```), if you know it that'll be great. All other callers for ```EditWindow``` have been tested working.