Crash on modifying SpeedTree LOD scale

Steps to repro:
1. Open attached project > Project Window > Open "SpeedTreeCustomizationMain" folder.
2. Select "Broadleaf_Desktop.spm" tree.
3. In the Inspector change LOD scale.
4. Enter Play Mode.
5. "Unapplied import settings" pop up will appear.
6. Click on Apply.
7. Observe the crash.

Expected Result:
Changes should apply on prefab and enter in Game mode.

Reproducible on:
2018.1.0a3, 2018.1.0a4, 2018.1.0a5, 2018.1.0a6, 2018.1.0a7, 2018.1.0b1

Works fine on:
2017.3.0f3, 2018.1.0a1, 2018.1.0a2.

Environment:
Occurring only on MAC.

Frequency:
Always