When Objects prefab is Merged and enter in the Prefab Asset Mode then it causes the editor to throw IndexOutOfRangeException, refer the attached video.

Steps to repro:
1. Create a new project.
2. Window > Package Manager > ProBuilder > Install
3. Tools > ProBuilder > ProBuilder Window
4. Create 2 new shape object
5. Make prefab of 2 New Shape object
6. Select 2 Object in Hierarchy > ProBuilder Window > Merge Objects
7. Select Merge Object > Open Prefab Asset object

Actual Result:
IndexOutOfRangeException is thrown when entering Prefab Mode Assets of Merge Objects

Expected Result:
Merge Objects enter in Prefab Asset Mode without any exception is thrown in the console

Occurring in:
2020.2.0a11, 2020.1.0b9, 2020.1.0a23 with package version: preview.9 - 4.3.0, 4.2.3

Working fine in:
2020.1.0a22, 2019.3.14f1 with package version: preview.9 - 4.3.0, 4.2.3

Environment:
Occurs only on Windows 10

Stack trace:
IndexOutOfRangeException: Index was outside the bounds of the array.
UnityEngine.ProBuilder.Math.Normal (UnityEngine.ProBuilder.ProBuilderMesh mesh, UnityEngine.ProBuilder.Face face) (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/runtime/core/math.cs:658)
UnityEngine.ProBuilder.Projection.PlanarProject (UnityEngine.ProBuilder.ProBuilderMesh mesh, UnityEngine.ProBuilder.Face face, UnityEngine.Vector3 projection) (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/runtime/core/projection.cs:128)
UnityEngine.ProBuilder.UvUnwrapping.Unwrap (UnityEngine.ProBuilder.ProBuilderMesh mesh, UnityEngine.ProBuilder.Face face, UnityEngine.Vector3 projection) (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/runtime/meshoperations/uv/uvunwrapping.cs:12)
UnityEngine.ProBuilder.ProBuilderMesh.RefreshUV (System.Collections.Generic.IEnumerable`1[T] facesToRefresh) (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/runtime/core/probuildermeshfunction.cs:460)
UnityEngine.ProBuilder.ProBuilderMesh.Refresh (UnityEngine.ProBuilder.RefreshMask mask) (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/runtime/core/probuildermeshfunction.cs:376)
UnityEngine.ProBuilder.ProBuilderMesh.MakeUnique () (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/runtime/core/probuildermeshfunction.cs:333)
UnityEditor.ProBuilder.EditorUtility.SynchronizeWithMeshFilter (UnityEngine.ProBuilder.ProBuilderMesh mesh) (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/editor/editorcore/editorutility.cs:232)
UnityEditor.ProBuilder.HierarchyListener.HierarchyWindowChanged () (at library/packagecache/com.unity.probuilder@4.3.0-preview.9/editor/editorcore/hierarchylistener.cs:66)
UnityEditor.EditorApplication.Internal_CallHierarchyHasChanged () (at <89a088b11b1f446fb543c196a8473632>:0)