Reproduction steps:
1. Open unity editor
2. Create a new Project
3. In the Project window right-click and select Import New Asset...
4. Select BANCO60.fbx
5. In Inspector tab press on a Generate Litmap UVs checkbox
6. Press Apply

Expected result: changes are applied
Actual result: UnwralCL.exe crash

Reproducible with: 2017.3.0a4, 2017.3.0a5, 2017.3.0a7, 2017.3.0b5
Not reproducible with: 2017.2.0f1, 2017.2.0f3, 2017.3.0a3, 2018.1.0a1
Regression first introduced in: 2017.3.0a4

Notes: if both Swap UVs and Generate Lightmap UVs are selected UnwralCL.exe does not crash
if in the Advanced options Hard Angle is set to 0 editor freezes for half a minute

Fixed in 2018.1.0a1