When the user duplicates an audio group, the audio group is created outside the audio mixer. If the audio mixer is removed from the project and the duplicated audio group is used as an output source for a audio file, the editor crashes on play.

Steps to Reproduce:
1. Project Window > right click > create audio mixer
2. Project Window > expand audio mixer > duplicate the master group
3. Observe that the duplicate master group is created outside the audio mixer.
4. Delete the audio mixer > Go to scene > create an audio source > attach duplicated master group as output
5. Run the scene & observe the crash

Expected Result:
On duplicating an audio group, a new audio mixer should be created.

Actual Result:
Null reference exception/Crash

Working fine on:
2017.3.0f3, 2017.2.0f3

Reproducible on:
2018.1.0b1, 2018.1.0a7, 2018.1.0a5, 2018.1.0a1

Environment :
Occurring on Windows and Mac.