How to reproduce:
1. Open the attached project “VR Chat World.zip“
2. Open Scene “main“ (Assets/Scenes)
3. Select the “Cube (1)“ GameObject in the Hierarchy
4. Open ProBuilder (Tools>ProBuilder>ProBuilder Window)
5. Select any vertical edge of the GameObject
6. Press “Insert Edge Loop“ in the ProBuilder window

Expected result: Edge loop is inserted on to the GameObject
Actual result: “NullReferenceException: Object reference not set to an instance of an object“ error in the Console

Reproducible with: 5.0.4, 5.0.6 (2020.3.45f1, 2021.3.18f1, 2022.2.6f1, 2023.1.0b3,2023.2.0a2)

Reproduced on: macOS 13.0.1 (Intel)