Steps:
1. Open the project in 2018.3 b12
2. Double click on each prefab in the Prefabs folder to confirm they all work
3. Close the prefab view (exit prefab editing)
4. Build the project
5. Double click on each prefab in the Prefabs folder again

Expected Result
Should still be able to edit the prefabs

Actual Result
Opening Prefab Mode failed: Could not detect a Prefab root after loading 'Assets/Prefabs/Prefab_Empty.prefab'.