How to reproduce:
1. Create and open a new project
2. Open the Package Manager window (Window -> Package Manager)
3. Install the ProBuilder package
4. In the Hierarchy, create a ProBuilder door (Right Click -> ProBuilder -> Door)
5. Select the “Door” GameObject, and in the Inspector, right-click the ProBuilder MeshFilter and select "Export"
6. Press "Export"
7. Observe the result

Expected result: A file explorer window opens, allowing the mesh to be exported
Actual result: An IndexOutOfRangeException appears in the Console

Reproducible with: 6.0.6 (6000.0.0f1, 6000.0.55f1, 6000.2.0f1, 6000.3.0a4)
Not reproducible with 6.0.5 (6000.0.0f1, 6000.0.55f1)

Reproducible on: Windows 11
Not reproducible on: No other environments tested

Note: Does not reproduce on all ProBuilder meshes. Cube, stairs, torus, and cone, for example, do not reproduce the issue