Steps to reproduce:
1. Create a new project
2. Make sure Use new Hierarchy Window option is enabled (Unity > Settings… > General)
3. Install Entities package
4. Create Two Sub Scenes in the Hierarchy (New Sub Scene > Empty Scene)
5. Select New Sub Scene GameObject
6. Drag it in “New Sub Scene” scene in Hierarchy window
7. Select “New Sub Scene 2” GameObject and drag it to the “New Sub Scene” scene in Hierarchy window
8. Save changes by pressing Command+S
9. Observe Console window

Actual results: Errors are thrown in the Console window:
“[Worker0] InvalidOperationException: Cannot create a new scene additively with an untitled scene unsaved.
UnityEditor.SceneManagement.EditorSceneManager.NewScene (UnityEditor.SceneManagement.NewSceneSetup setup, UnityEditor.SceneManagement.NewSceneMode mode) <0x17fe57cd0 + 0x0004f> in <1ee54be0ec644ade9c054f148c52822e>:0
Unity.Scenes.SubScene.UnloadScene () (at ./Library/PackageCache/com.unity.entities@882760f0ebad/Unity.Scenes/SubScene.cs:414)
Unity.Scenes.SubScene.OnDestroy () (at ./Library/PackageCache/com.unity.entities@882760f0ebad/Unity.Scenes/SubScene.cs:424)
UnityEditor.SceneManagement.EditorSceneManager:CloseScene(Scene, Boolean)
Unity.Scenes.Editor.SubSceneImporter:OnImportAsset(AssetImportContext) (at ./Library/PackageCache/com.unity.entities@882760f0ebad/Unity.Scenes.Editor/SubSceneImporter.cs:204)
UnityEditor.AssetImporters.ScriptedImporter:GenerateAssetData(AssetImportContext) (at /Users/bokken/build/output/unity/unity/Modules/AssetPipelineEditor/Public/ScriptedImporter.cs:25)”

Expected results: No errors are thrown in the Console window

Reproducible with versions: 6000.3.3f1, 6000.4.0b3, 6000.5.0a4
Can’t test with versions: 6000.0.65f1 (No new Hierarchy window)

Tested on (OS): macOS Silicon Tahoe 26.0.1 (M1)