Steps to reproduce:

1. Open Unity;
2. Create new project;
3. In the 'Project' tab right mouse click -> Create -> Material;
4. Click anywhere in the 'Project' tab;
5. Select recently created material;
6. Observe 'Project' tab and preview window in the Inspector.

Actual result:
Material's preview and icon isn't displayed (see attached screenshot).

Regression introduced on 5.4.0b5.

Expected result:
Material's preview and icon should be displayed.

Notes:
- Reproduced in OSX 10.11.3 (Glcore) and Windows 10 (DX11);
- Reproduces in 5.4.0b5;
- Doesn't reproduce in 5.4.0b4;
- Doesn't reproduce for 3d models (like .spm files).