When trying to save from the UI builder outside of the project directory this exception will happen and nothing else gets reported to the user. I think it would be great if the error message could be improved.
{code:java}
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 C:/code/unity_trunk/External/MirroredPackageSources/com.unity.ui.builder/Editor/Utilities/VisualTreeAssetExtensions/VisualTreeAssetExtensions.cs:44)
Unity.UI.Builder.BuilderDocumentOpenUXML.PostSaveToDiskChecksAndFixes (System.String newUxmlPath, System.Boolean needsFullRefresh) (at C:/code/unity_trunk/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Document/BuilderDocumentOpenUXML.cs:1019)
Unity.UI.Builder.BuilderDocumentOpenUXML.SaveNewDocument (UnityEngine.UIElements.VisualElement documentRootElement, System.Boolean isSaveAs, System.Boolean& needsFullRefresh, System.String manualUxmlPath) (at C:/code/unity_trunk/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Document/BuilderDocumentOpenUXML.cs:466)
Unity.UI.Builder.BuilderDocument.SaveNewDocument (UnityEngine.UIElements.VisualElement documentRootElement, System.Boolean isSaveAs, System.Boolean& needsFullRefresh, System.String manualUxmlPath) (at C:/code/unity_trunk/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Document/BuilderDocument.cs:272)
Unity.UI.Builder.BuilderToolbar.SaveDocument (System.Boolean isSaveAs) (at C:/code/unity_trunk/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Toolbar/BuilderToolbar.cs:376)
Unity.UI.Builder.BuilderToolbar.<SetUpFileMenu>b__45_5 (UnityEngine.UIElements.DropdownMenuAction a) (at C:/code/unity_trunk/External/MirroredPackageSources/com.unity.ui.builder/Editor/Builder/Toolbar/BuilderToolbar.cs:508)
UnityEngine.UIElements.DropdownMenuAction.Execute () (at C:/code/unity_trunk/ModuleOverrides/com.unity.ui/Core/DropdownMenu.cs:189)
UnityEditor.UIElements.EditorMenuExtensions+<>c__DisplayClass0_0.<PrepareMenu>b__0 () (at C:/code/unity_trunk/Editor/Mono/UIElements/EditorMenuExtensions.cs:35)
UnityEditor.GenericMenu.CatchMenu (System.Object userData, System.String[] options, System.Int32 selected) (at C:/code/unity_trunk/Editor/Mono/GUI/GenericMenu.cs:127) {code}