On performing Undo and Redo operation with the Smooth Group Editor window then it causes the editor to throw NullReferenceException, Refer attached video.

Steps to repro:
1. Create a new project.
2. Window > Package Manager > ProBuilder > Install
3. Create a new shape and select it
4. ProBuilder window > Smooth Group Editor window > Change the properties
5. Edit > Undo set Smoothing Group > Undo Create Shape
6. Edit > Redo Create Object

Actual Result:
NullReferenceException is thrown on performing Undo and Redo operation with Smooth Group Editor window

Expected Result:
Undo and Redo operation is performed gracefully

Occurring on:
2021.1.0a1, 2020.2.0b7 with package version 4.4.0

Working fine on:
2020.2.0b7 with package version 4.3.1

Environment:
Occurring only on Windows 10

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.ProBuilder.SmoothGroupEditor+SmoothGroupData.RebuildNormalsMesh (UnityEngine.ProBuilder.ProBuilderMesh pb) (at Library/PackageCache/com.unity.probuilder@4.4.0/Editor/EditorCore/SmoothGroupEditor.cs:108)
UnityEditor.ProBuilder.SmoothGroupEditor+SmoothGroupData.Rebuild (UnityEngine.ProBuilder.ProBuilderMesh pb) (at Library/PackageCache/com.unity.probuilder@4.4.0/Editor/EditorCore/SmoothGroupEditor.cs:54)
UnityEditor.ProBuilder.SmoothGroupEditor+SmoothGroupData..ctor (UnityEngine.ProBuilder.ProBuilderMesh pb) (at Library/PackageCache/com.unity.probuilder@4.4.0/Editor/EditorCore/SmoothGroupEditor.cs:46)
UnityEditor.ProBuilder.SmoothGroupEditor.OnSelectionChanged () (at Library/PackageCache/com.unity.probuilder@4.4.0/Editor/EditorCore/SmoothGroupEditor.cs:394)
UnityEditor.ProBuilder.MeshSelection.OnObjectSelectionChanged () (at Library/PackageCache/com.unity.probuilder@4.4.0/Editor/EditorCore/MeshSelection.cs:200)
UnityEditor.Selection.Internal_CallSelectionChanged () (at <859685c8158c480a8d135de9619162c9>:0)
UnityEditor.EditorApplication:Internal_CallGlobalEventHandler()