How to reproduce:
1. Open the attached project
2. In Project window, select "cloisterGarden_noFountain"
3. Open Inspector window and choose "cloisterGarden..." Model settings
4. Enable "Generate Lighmap UV's" setting and press apply (re-enable if its already enabled)

Expected result: Model gets re-imported with the new settings
Actual result: Crash occurs on:

========== OUTPUTTING STACK TRACE ==================

0x0000000140F8098C (Unity) UnwrapImpl::GenerateSecondaryUVSet
0x00000001410BCA2E (Unity) GenerateMeshData
0x00000001410BB478 (Unity) GenerateMeshData
0x000000014111143D (Unity) ModelImporter::InstantiateImportMesh
0x0000000141111979 (Unity) ModelImporter::InstantiateImportNode
0x0000000141112BF3 (Unity) ModelImporter::InstantiateImportNode
0x00000001411085E1 (Unity) ModelImporter::GenerateAll
0x000000014110B20D (Unity) ModelImporter::GenerateAssetData
....

Fixed in - 2018.2.0b6

Regression introduced in - 2017.2.0a1
Reproducible with - 2018.1.0b1, 2017.3.0p1, 2017.2.1p1, 2017.2.0a1
Not reproducible with - 2017.1.2p4