To reproduce:
1.Open users attached project "Empty Project.zip"
2.Open QuickLod > DemoScene > "MainDemoScene.unity" scene
3.Go back to assets folder
4.Right click on QuickLod folder and press Reimport

Expected result: Assets are reimported
Actual result: Unity crashes

Reproduced on: 5.6.1p1
Not reproduced on: 5.5.3p4
Regression introduced in: 5.6.0a3

Also reproduced on: 5.6.0a3 5.6.0b4 5.6.0b7 5.6.0f1 5.6.0f3 5.6.1f1 2017.1.0b6 2017.2.0a1
Also not reproduced on: 5.5.0f3 5.5.3p2 5.6.0a1 5.6.0a2

Notes: tested on MAC X OS, same result.