Steps to reproduce:
1. Create new audio mixer
2. Create new audio group
3. Add Audio Source to the scene
4. Try selecting audio output group through output group object selector

Actual: Audio output groups are shown as a list
Expected: Audio output groups should be shown with a tree structure

Reproducible: 2017.3.0b3, 2018.1.0b9, 2018.2.0a2
Not reproducible: 2017.3.0a7