How to reproduce:
1. Open the attached project
2. Go to Assets Scenes/EmptySeries/ and open "EmptySeries-Complete.unity" Scene
3. Go to Tools -> ProBuilder -> ProBuilder Window and create a new shape by clicking on "+" near the "New Shape" -> Build
4. Open ProBuilder and click on "Material Editor"
5. Go to Assets Content\IntroToStorytelling\Environment\Rock\Materials drag and drop any material into the Quick Material slot (if it's not reproducible, try to drop different materials)

Expected result: Material is dropped
Actual result: Material editor goes black

Reproducible: 2018.3.5f1, 2019.1.0b3, 2019.2.0a5
Can't reproduce: 2018.3.0a1 - 2018.3.0b9, 2019.1.0a1-2019.1.0a5, because the ProBuilder window selection is missing with

Notes:
- ProBuilder package is available in 2018.1 and later