Steps to reproduce:
1. Create new Unity project using Universal 3D Template
2. Install Entities package via Package Manager window
3. In Hierarchy create new Sub Scene
4. Open Preferences window and go to Entities section
5. Enable “Show Advanced Worlds”
6. Open Systems window (Window > Entities > Systems)
7. Click on “Default World” dropdown
8. Select “LoadingWorld3”
9. Observe the Console window

Actual results: “ArgumentException: WorldProxy for given world LoadingWorld3 does not exist or is null” error is thrown in the Console window

Expected results: No errors are thrown in the Console window

Reproducible with versions: 6000.5.0a6 (07a4e6f4c5c6), 6000.5.6f1, 6000.6.0b6, 6000.7.0a3

Not reproducible with versions: 6000.3.21f1, 6000.4.12f1, 6000.5.0a5 (9b63d0bc55db)

Tested on (OS): macOS Tahoe 26.5.2 (M1)
{code:java}
ArgumentException: WorldProxy for given world LoadingWorld3 does not exist or is null
Unity.Entities.Editor.WorldProxyManager.GetWorldProxyForGivenWorld (Unity.Entities.World world) (at ./Library/PackageCache/com.unity.entities@0633f08e59db/Unity.Entities.Editor/SystemSchedule/PlayerLoop/WorldProxyManager.cs:23)
Unity.Entities.Editor.SystemScheduleWindow.OnWorldSelected (Unity.Entities.World world) (at ./Library/PackageCache/com.unity.entities@0633f08e59db/Unity.Entities.Editor/SystemSchedule/SystemScheduleWindow.cs:534)
Unity.Entities.Editor.DOTSEditorWindow.Update () (at ./Library/PackageCache/com.unity.entities@0633f08e59db/Unity.Entities.Editor/Common/DOTSEditorWindow.cs:225)
UnityEditor.HostView.SendUpdate () (at /Users/bokken/build/output/unity/unity/Editor/Mono/HostView.cs:719)
UnityEditor.EditorApplication.Internal_CallUpdateFunctions () (at /Users/bokken/build/output/unity/unity/Editor/Mono/EditorApplication.cs:413)
UnityEngine.StackTraceUtility:ExtractStringFromExceptionInternal(Object, String&, String&) (at /Users/bokken/build/output/unity/unity/Runtime/Export/Scripting/StackTrace.cs:50)
{code}