When Sprite Atlas asset is selected in the Project window and "Sprite Atlas V2(Experimental) - Enabled" Sprite Packer Mode is selected in the Editor settings window then it causes the editor to throw NullReferenceException continuously, Refer to attached video.

Steps to repro:
1. Create a new project with a 2D Template
2. Project > Create > 2D > Sprite Atlas
3. Edit > Project Settings > Editor > Sprite Packer > Sprite Atlas V2(Experimental) - Enabled
4. Project > New Sprite Atlas > Select

Actual Result: NullReferenceException is thrown when Sprite Atlas asset is selected in the Project window and "Sprite Atlas V2(Experimental) - Enabled" Sprite Packer Mode is selected in the Editor settings window

Expected result: "Sprite Atlas V2(Experimental) - Enabled" Sprite Packer Mode is selected gracefully

Occurring in:
2021.1.0a7, 2021.1.0a6

Working fine in:
2021.1.0a5, 2020.2.0b13, 2020.1.12f1

Environment:
Windows10 and macOS 10.15

Stack trace:
NullReferenceException: Alpha Dilation: SerializedProperty is null
UnityEditor.EditorGUI.BeginPropertyInternal (UnityEngine.Rect totalPosition, UnityEngine.GUIContent label, UnityEditor.SerializedProperty property) (at <2e08b753a9374ad482284720aea4656b>:0)
UnityEditor.EditorGUI.BeginProperty (UnityEngine.Rect totalPosition, UnityEngine.GUIContent label, UnityEditor.SerializedProperty property) (at <2e08b753a9374ad482284720aea4656b>:0)
UnityEditor.U2D.SpriteAtlasImporterInspector.HandleBoolToIntPropertyField (UnityEditor.SerializedProperty prop, UnityEngine.GUIContent content) (at <2e08b753a9374ad482284720aea4656b>:0)
UnityEditor.U2D.SpriteAtlasImporterInspector.HandleMasterSettingUI () (at <2e08b753a9374ad482284720aea4656b>:0)
UnityEditor.U2D.SpriteAtlasImporterInspector.OnInspectorGUI () (at <2e08b753a9374ad482284720aea4656b>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass59_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <a3d19fcb75d5423995f038fd978f8223>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)