GridView filtering/sorting null enum properties.

[dmitriy]

Hi!
It is seems that gridview doesn't separate null value and enum's first value while filtering or sorting.

Example: 

MainWindow.xaml.cs : https://codeshare.io/aJb3dK

MainWindow.xaml:  https://codeshare.io/5Odrk0

Try to filter column by choosing Item 1.