Editor thrown NullReferenceException on enabling the "Open files in Editor once created" while creating UI Toolkit window refer to the attached video

Steps to repro:
1. Create a new Project.
2. Project > Create > UI Toolkit >Editor window
3. Add C#, UXML, USS file name
4. Enabling the "Open files in Editor once created"
5. Click on the Confirm button

Actual result: NullReferenceException appear on enabling the "Open files in Editor once created" while creating UI Toolkit window

Expected result: "UI Toolkit window" create and open gracefully

Reproducible in:
2021.1.0a7, 2021.1.0a2, 2020.2.0b13, 2020.2.0b10

Working Fine in:
2021.1.0a1, 2020.2.0b9

Environment: Occurs only on Linux

Verified on Ubuntu 18.04 LTS

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.UIElements.StyleSheets.StyleSelectorHelper.FastLookup (System.Collections.Generic.IDictionary`2[TKey,TValue] table, System.Collections.Generic.List`1[T] matchedSelectors, UnityEngine.UIElements.StyleMatchingContext context, System.String input, UnityEngine.UIElements.StyleSheets.SelectorMatchRecord& record) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/StyleSheets/StyleSelectorHelper.cs:199)
UnityEngine.UIElements.StyleSheets.StyleSelectorHelper.FindMatches (UnityEngine.UIElements.StyleMatchingContext context, System.Collections.Generic.List`1[T] matchedSelectors) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/StyleSheets/StyleSelectorHelper.cs:226)
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.TraverseRecursive (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/VisualTreeStyleUpdater.cs:236)
UnityEngine.UIElements.StyleSheets.HierarchyTraversal.Recurse (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/HierarchyTraversal.cs:24)
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.TraverseRecursive (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/VisualTreeStyleUpdater.cs:260)
UnityEngine.UIElements.StyleSheets.HierarchyTraversal.Recurse (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/HierarchyTraversal.cs:24)
UnityEngine.UIElements.VisualTreeStyleUpdaterTraversal.TraverseRecursive (UnityEngine.UIElements.VisualElement element, System.Int32 depth) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/VisualTreeStyleUpdater.cs:260)
UnityEngine.UIElements.StyleSheets.HierarchyTraversal.Traverse (UnityEngine.UIElements.VisualElement element) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/HierarchyTraversal.cs:11)
UnityEngine.UIElements.VisualTreeStyleUpdater.ApplyStyles () (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/VisualTreeStyleUpdater.cs:108)
UnityEngine.UIElements.VisualTreeStyleUpdater.Update () (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/VisualTreeStyleUpdater.cs:91)
UnityEngine.UIElements.VisualTreeUpdater.UpdateVisualTreePhase (UnityEngine.UIElements.VisualTreeUpdatePhase phase) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/VisualTreeUpdater.cs:154)
UnityEngine.UIElements.Panel.ValidateLayout () (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/Panel.cs:880)
UnityEngine.UIElements.UIElementsUtility.DoDispatch (UnityEngine.UIElements.BaseVisualElementPanel panel) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/UIElementsUtility.cs:459)
UnityEngine.UIElements.UIElementsUtility.UnityEngine.UIElements.IUIElementsUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& eventHandled) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/UIElementsUtility.cs:209)
UnityEngine.UIElements.UIEventRegistration.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/UIElementsUtility.cs:74)
UnityEngine.UIElements.UIEventRegistration+<>c.<.cctor>b__1_2 (System.Int32 i, System.IntPtr ptr) (at /home/bokken/buildslave/unity/build/External/MirroredPackageSources/com.unity.ui/Core/UIElementsUtility.cs:28)
UnityEngine.GUIUtility.ProcessEvent (System.Int32 instanceID, System.IntPtr nativeEventPtr, System.Boolean& result) (at /home/bokken/buildslave/unity/build/Modules/IMGUI/GUIUtility.cs:189)
UnityEditor.EditorWindow:GetWindow()
aaaa:ShowExample() (at Assets/Editor/aaaa.cs:12)
UnityEditor.HostView:OnEnable() (at /home/bokken/buildslave/unity/build/Editor/Mono/HostView.cs:152)