To reproduce:

1. Download attached project "workbench-2018.2.zip" and open in Unity
2. Open "scene" scene
3. Press Tools > ProBuilder > ProBuilder Window
4. Select "New Game Object" in Hierarchy
5. Switch to Edge Selection mode
6. Select two opposing edges
7. Click "Connect Edges" in the Probuilder window

Expected result: The Edges should be split at their midpoints and a new Edge added between the two new vertices
Actual result: Either a message appears saying "0 edges connected," or an exception is thrown

Notes:
- This issue appears both on Windows and OSX
- An Exception is shared in Comments section

Reproduced on Package version 3.0.0 and 3.0.9

Reproduced on Unity 2018.1.9f2, 2018.2.7f1 and 2018.3.0a11