Null Reference exception is thrown on changing scale factor of the screen from Window's display settings.

Steps to reproduce:
1. Create a new project.
2. Go to Display Settings of windows and change scale 2-3 times randomly. (ex. change from 100% to 150% OR 175% to 125%)

Note: Appears 2 out of 10 times.

Actual Result:
Null Reference exception is thrown in the console on changing scale factor of the screen from Window's display settings.

Expected Result:
Scale factor changes.

Occurring On:
2019.1.0a10

Working fine On:
2019.1.0a9, 2018.3.0b12

Environment:
Windows Only

Stack Trace:
NullReferenceException: Object reference not set to an instance of an objectUnityEngine.UIElements.VisualElement.AddStyleSheetPath (System.String sheetPath) (at C:/buildslave/unity/build/Modules/UIElements/VisualElementStyleAccess.cs:93)UnityEditor.UIElements.UIElementsEditorUtility.AddDefaultEditorStyleSheets (UnityEngine.UIElements.VisualElement p) (at C:/buildslave/unity/build/Editor/Mono/UIElements/UIElementsEditorUtility.cs:22)UnityEditor.EditorWindow.CreateRoot[DefaultEditorMode] () (at C:/buildslave/unity/build/Editor/Mono/EditorWindow.cs:122)UnityEditor.EditorWindow.GetRootElement[DefaultEditorMode] (Boolean createIfNull) (at C:/buildslave/unity/build/Editor/Mono/EditorWindow.cs:82)UnityEditor.EditorWindow.GetRootElement[DefaultEditorMode] (Boolean createIfNull) (at C:/buildslave/unity/build/Editor/Mono/EditorWindow.cs:86)Unity.Experimental.EditorMode.EditorModes.GetRootElement[DefaultEditorMode] (UnityEditor.EditorWindow window) (at C:/buildslave/unity/build/Editor/Mono/EditorMode/EditorModes.cs:455)Unity.Experimental.EditorMode.EditorModes+ModeGenericHelper`1[Unity.Experimental.EditorMode.DefaultEditorMode].GetRoot (UnityEditor.EditorWindow window) (at C:/buildslave/unity/build/Editor/Mono/EditorMode/EditorModes.cs:28)Unity.Experimental.EditorMode.EditorModes.GetRootElement (UnityEditor.EditorWindow window) (at C:/buildslave/unity/build/Editor/Mono/EditorMode/EditorModes.cs:215)UnityEditor.HostView.UpdateViewMargins (UnityEditor.EditorWindow view) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:71)UnityEditor.HostView.SetActualViewPosition (Rect newPos) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:91)UnityEditor.DockArea.SetActualViewPosition (Rect newPos) (at C:/buildslave/unity/build/Editor/Mono/GUI/DockArea.cs:617)UnityEditor.HostView.SetPosition (Rect newPos) (at C:/buildslave/unity/build/Editor/Mono/HostView.cs:83)UnityEditor.View.set_position (Rect value) (at C:/buildslave/unity/build/Editor/Mono/View.cs:119)UnityEditor.SplitView.SetupRectsFromSplitter () (at C:/buildslave/unity/build/Editor/Mono/GUI/SplitView.cs:112)UnityEditor.SplitView.Reflow () (at C:/buildslave/unity/build/Editor/Mono/GUI/SplitView.cs:165)UnityEditor.SplitView.SetPosition (Rect newPos) (at C:/buildslave/unity/build/Editor/Mono/GUI/SplitView.cs:742)UnityEditor.View.set_position (Rect value) (at C:/buildslave/unity/build/Editor/Mono/View.cs:119)UnityEditor.MainView.SetPosition (Rect newPos) (at C:/buildslave/unity/build/Editor/Mono/GUI/MainView.cs:26)UnityEditor.View.set_position (Rect value) (at C:/buildslave/unity/build/Editor/Mono/View.cs:119)UnityEditor.ContainerWindow.OnResize () (at C:/buildslave/unity/build/Editor/Mono/ContainerWindow.cs:244)