How to reproduce:
1. Open the attached project named Case_1164940
2. Focus the Scene view
3. Press G

Expected result: No errors are thrown in the Editor
Actual result: NullReferenceException is thrown

Reproducible with: 2019.1.0a14, 2019.1.8f1, 2019.2.0b7, 2019.3.0a7
Not reproducible with: 2018.4.3f1
Could not test with: 2017.4.29f1(No package manager), 2019.1.0a1 - 2019.1.0a13(Editor throws errors right after importing ProBuilder)

Reproducible with package versions: 4.0.4, 4.0.5
Not reproducible with package version: 3.10.1, 4.0.2, 4.0.3

Notes:
1. The issue only happens when the Scene view is focused
2. If the ProBuilder window is opened the issue does not reproduce(Showed in the Track_1164940_V1)