While creating new ProBuilder mesh child with inverted normals on selecting "Gen Shadow Obj" tool from ProBuilder window, NullReferenceException is thrown, refer attached video and Stack Trace.

Steps to repro:
1. Open attached project.
2. Window > Package Manager > ProBuilder > Install
3. Tools > ProBuilder > ProBuilder Window
4. ProBuilder > Open the "Gen Shadow Obj" tool button

Actual Result:
NullReferenceException is thrown in the console on selecting "Gen Shadow Obj" tool from ProBuilder window.

Expected Result:
On selecting "Gen Shadow Obj" tool, New ProBuilder mesh child object is created with inverted normals.

Reproducible in:
2020.1.0a19, 2019.3.0f6, 2018.1.9f2

Probuilder package version found: 4.2.1, 4.1.0, 3.0.0

Environment:
Windows 10 & macOS 10.14