Steps to reproduce:
1. Create a new Unity project
2. Open Edit->Undo History
3. Make sure an undo event is selected and press Escape key

Expected result: Nothing happens
Actual result: NullReferenceException error is thrown
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.UndoHistoryWindow.OnUndoSelectionChange (System.Collections.Generic.IEnumerable`1[T] selectedItems) (at /Users/bokken/buildslave/unity/build/Editor/Mono/Undo/UndoHistoryWindow.cs:201)
UnityEngine.UIElements.BaseVerticalCollectionView.NotifyOfSelectionChange () (at /Users/bokken/buildslave/unity/build/ModuleOverrides/com.unity.ui/Core/Controls/BaseVerticalCollectionView.cs:1341)
UnityEngine.UIElements.BaseVerticalCollectionView.ClearSelection () (at /Users/bokken/buildslave/unity/build/ModuleOverrides/com.unity.ui/Core/Controls/BaseVerticalCollectionView.cs:1354)

Reproduced with: 2022.1.0a8, 2022.1.0a4 (Undo History was added in 2022.1.0a4)