To reproduce:

1. Download attached project "SceneExperiements.zip" and open in Unity
2. Open "Lobby" scene
3. Press Tools > ProBuilders > Editors > New Poly Shape
4. Click in Scene view 4 times to create a Square Poly Shape
5. Observe that when you close the Shape, the Errors in the Console I listed in Comments section is repeated hundreds of times until you extrude the Shape, at which point the Error messages stop

Notes:
- This issue appears both on Windows and OSX
- Errors and reproduction video is shared in Comments sections
- Workaround - Setting Unity Editor layout to any of the built-in Editor layout fixes the issue

Reproduced on Package version 3.0.0 and 3.0.9

Reproduced on Unity 2018.1.9f2, 2018.2.6f1 and 2018.3.0a10