Steps to reproduce:
1. Create new Unity project using Universal 3D Template
2. Open package manager window and install Entities package
3. In Hierarchy window create New Sub Scene
4. Right-click on New Sub Scene (under Global Volume) and create Button element in Hierarchy (GameObject > UI Toolkit > Standard Elements > Button)
5. Press “Continue” in Open visual element editing stage pop-up
6. Save “NewUXMLTemplate”
7. Go back to Hierarchy window by pressing on “<“ button on “NewUXMLTemplate”
8.Save changes
9. Observe the Console window

Actual results: “NullReferenceException: Object reference not set to an instance of an object” error is thrown in the Console window

Expected results: No errors are thrown in the Console window

Reproducible with versions: 6000.6.0b3, 6000.7.0a2

Can’t test with versions: 6000.5.3f1 and lower (No such feature to create UI Elements in Hierarchy window)

Tested on (OS): macOS Tahoe 26.5.1 (M1)

NullReferenceException: Object reference not set to an instance of an object
Unity.Entities.Editor.HierarchyEntityHandler.Dispose (System.Boolean disposing) (at ./Library/PackageCache/com.unity.entities@572553326ed5/Unity.Entities.Editor/Hierarchy/HierarchyEntityHandler.cs:86)
Unity.Hierarchy.HierarchyNodeTypeHandlerBase.InvokeDispose (System.IntPtr handlePtr) (at /Users/bokken/build/output/unity/unity/Modules/HierarchyCore/Managed/HierarchyNodeTypeHandlerBase.cs:345)
UnityEngine.StackTraceUtility:ExtractStringFromExceptionInternal(Object, String&, String&)
Unity.Hierarchy.Hierarchy:Destroy(IntPtr)
Unity.Hierarchy.Hierarchy:Dispose(Boolean) (at /Users/bokken/build/output/unity/unity/Modules/HierarchyCore/ScriptBindings/Hierarchy.bindings.cs:151)
Unity.Hierarchy.Hierarchy:Dispose() (at /Users/bokken/build/output/unity/unity/Modules/HierarchyCore/ScriptBindings/Hierarchy.bindings.cs:142)
Unity.Hierarchy.Editor.HierarchyWindow:OnStageChanged(Stage, Stage) (at /Users/bokken/build/output/unity/unity/Modules/HierarchyEditor/Managed/HierarchyWindow.cs:847)
UnityEditor.SceneManagement.StageNavigationManager:NavigateBack(ChangeType)
Unity.Hierarchy.Editor.StageNavigationView:OnBackPressed(PointerUpEvent) (at /Users/bokken/build/output/unity/unity/Modules/HierarchyEditor/Managed/StageNavigationView.cs:75)
UnityEditor.GUIView:ProcessEvent(EntityId, IntPtr, Boolean&) (at /Users/bokken/build/output/unity/unity/Editor/Mono/GUIView.cs:70)