.uxml file does not create after disabling Validation in .uss file Importer settings
Workaround: Project Winodw > Right-click > Refresh. (.uxml file will appear)

Steps to repro:
1. Create a new Project.
2. Window > UI Toolkit > UI Builder.
3. StyleSheets > Click on "+" > Create new USS > Test1.
4. Project Window > Select Test1 > Enable "Disable Validation" checkbox > Apply > Save.
5. In Save UXML window > File name > Test2 > Save.

Actual Result:
-NullReferenceException errors in the console.
-Test2 asset is not showing in the Project window.

Expected Result:
Test2.uxml asset file appears in the Project window without any errors in the console.

Reproducible in:
2021.1.0a10, 2020.2.0f1, 2020.1.0f1, 2019.4.0f1 with Package version 1.0.0-preview.9.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
Unity.UI.Builder.VisualTreeAssetExtensions.DeepOverwrite (UnityEngine.UIElements.VisualTreeAsset vta, UnityEngine.UIElements.VisualTreeAsset other) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderDocumentOpenUXML.PostSaveToDiskChecksAndFixes (System.String newUxmlPath, System.Boolean needsFullRefresh) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderDocumentOpenUXML.SaveNewDocument (UnityEngine.UIElements.VisualElement documentRootElement, System.Boolean isSaveAs, System.Boolean& needsFullRefresh, System.String manualUxmlPath) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderDocumentOpenUXML.SaveUnsavedChanges (System.String manualUxmlPath, System.Boolean isSaveAs) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderDocumentOpenUXML.CheckForUnsavedChanges (System.Boolean assetModifiedExternally) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderDocument.CheckForUnsavedChanges (System.Boolean assetModifiedExternally) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderToolbar.LoadDocument (UnityEngine.UIElements.VisualTreeAsset visualTreeAsset, System.Boolean unloadAllSubdocuments, System.Boolean assetModifiedExternally) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderDocumentOpenUXML.OnPostProcessAsset (System.String assetPath) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderDocument.OnPostProcessAsset (System.String assetPath) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
Unity.UI.Builder.BuilderAssetPostprocessor.OnPostprocessAllAssets (System.String[] importedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromAssetPaths) (at <185bda1d3c3049d3b8a4852e0de3edf6>:0)
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <9577ac7a62ef43179789031239ba8798>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.MonoMethod.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <9577ac7a62ef43179789031239ba8798>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <9577ac7a62ef43179789031239ba8798>:0)
UnityEditor.AssetPostprocessingInternal.InvokeMethod (System.Reflection.MethodInfo method, System.Object[] args) (at <a19e86c7bf354115a4c6a5953c0575cb>:0)
UnityEditor.AssetPostprocessingInternal.PostprocessAllAssets (System.String[] importedAssets, System.String[] addedAssets, System.String[] deletedAssets, System.String[] movedAssets, System.String[] movedFromPathAssets) (at <a19e86c7bf354115a4c6a5953c0575cb>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)