Steps to Reproduce:
1. Open Unity editor
2. Create empty game object
3. Add Box collider
4. Create Phyiscs material in project tab
5. Assign phyiscs material to the box collider
6. Click on the select icon next to phyiscs material text box

Expected result: Physics material selection box should pop up
Actual result: exception is thrown when user clicks on Physics material selection icon

Reproduced in: 2017.3.0b1
Not reproduced in: 2017.3.0a7, 2017.2.0f1
Regression since: 2017.3.0b1