When a New Ploy Shape is created and exported then null reference exception thrown in the console

Steps to repro:
1. Create a new project with the "ProBuilder" package installed.
2. Tools > ProBuilder > ProBuilder Window
3. Dock 'ProBuilder' window.
4. Select New Poly Shape > Export

Actual Result :
NullReferenceException thrown in console

Expected Result:
No errors in the console.

Occurring with:
2020.2.0a18, 2020.1.0b16, 2019.4.4f1, 2019.3.16f1, 2018.4.25f1

Probuilder package version: 4.3.1, 4.2.3, 4.2.1.

Environment:
Windows10 and Mac 10.15