Steps to reproduce:
1. Open attached project
2. In project tab search for "Slime for filtering" and double click on it to open Hero Kit Editor
3. In Hero Kit Editor, expand States and select State 0: Slime
4. Notice that the preview window is empty (if it’s not, select something else in editor and return. It will be empty).

Expected result: Custom preview window should display a prefab model
Actual result Custom preview window is empty

Reproduced in: 2017.3.0a7, 2017.2.0f1, 2017.1.0b2
Not reproduced in: 2017.1.0b1, 5.6.3p3
Regression since: 2017.1.0b2