To reproduce:
1. Create Audio Mixer
2. Add new group to Master group
3. Add another new group to the previous group (Group hierarchy should look like this: Master -> New Group -> New Group)
4. Duplicate the child group of New Group
5. Add a new child to the Group.
6. Remove the newly added child.
7. Press Command + Z to undo/ in case of Windows - Ctr+Z
8. Editor crashes

Reproducible:[OSX] 2017.3.0a4, 2018.1.0a2

Reproducible with Windows: 2017.3.0b4,2017.3.0b5,2017.3.0b6,2017.3.0b7,2017.3.0b8, 2018.1.0a2,2018.1.0a3,2018.1.0a4

Stable with [Windows] 2017.2.0f3

Expected Result: Editor should not crash in case of undo actions.

Actual Result: After duplicating the child group and adding the new child to it and then removing it causes the editor crash.