Steps to reproduce:
1) Extract and open the project attached by user in Unity (TestScene.zip)
2) Go to Window > Lighting
3) Under 'Environment Lighting' select a different Skybox (any Material, Default for example)
4) On the first or second click on a material Unity crashes.

Reproduced on:
5.4.0b15, 5.4.0b10, on 5.3.4f1 reproduces about every second time when relaunching the project.

Not reproduced:
5.3.3f1, 5.3.2f1, 5.2.2f1,