To reproduce:
1. Import PolyWorld: Woodland Low Poly Toolkit package (https://www.assetstore.unity3d.com/en/#!/content/18572)
2. Open Demo_Woodland scene
3. Select SceneTerrain object and remove QT_Poly World Terrain component
4. Add QT_Poly World Terrain component to object
5. Set material to PW-Woodland on QT_Poly World Terrain component
6. Click Update All button
7. Create new folder in assets folder and select it

Actual: NullReferenceException and InvalidOperationException appears in console
Expected: Update should finish without errors

Reproducible: 5.6.0b6, 5.5.1f1
Not reproducible with 5.4.4p1