[Audio] Editor crash on resetting Audio Mixer group properties.

Steps to reproduce :
1. Create a new or open attached project.
2. Go to Window -> Audio -> Audio Mixer.
3. Click on '+' to create a new Mixer.
4. Enter Play mode.
5. Go to Master Group properties and reset via cogwheel.

Actual Result:
Editor crash on resetting Audio Mixer group properties.

Expected Result:
Unity should not crash and properties should reset.

Reproducible On: 2019.1.0a8, 2019.1.0a2, 2018.3.0b9, 2018.2.0f2, 2018.2.0b10

Working fine on: 2018.2.0b9

Environment:
Occurring on Windows
Working fine on Mac