Steps to reproduce:

1. Create a new project
2. Load the attached layout via Layout -> Load Layout from File...
3. Observe the open windows

Actual results: "Failed to load window" is displayed. Interacting with the window (Docking another window next to it, or redocking the affected window elsewhere) can lead to the window panel no longer being rendered and NullReferenceExceptions being thrown. This requires a Layout reset to fix.

Expected results: When loading a layout or creating a new project, any missing windows are omitted and not displayed

Reproducible with versions: 6000.3.9f1, 6000.3.18f1, 6000.4.0b9, 6000.5.3f1, 6000.6.0b3, 6000.7.0a2

Not reproducible with versions: 6000.0.79f1, 6000.3.8f1, 6000.4.0a1, 6000.4.0b8

Tested on (OS): macOS 26.5.1, Ubuntu 24

Notes:

• The issue also occurs when creating new projects. If the previously opened project contained any windows that will not be present in the new project, those windows will be displayed as "Failed to load" windows
• Full error message that is thrown after interacting with the window:

** NullReferenceException: Object reference not set to an instance of an object
UnityEditor.HostView.GetExtraButtonsWidth () (at /Users/bokken/build/output/unity/unity/Editor/Mono/HostView.cs:751)
UnityEditor.DockArea.OldOnGUI () (at /Users/bokken/build/output/unity/unity/Editor/Mono/GUI/DockArea.cs:383)
UnityEngine.UIElements.IMGUIContainer.DoOnGUI (UnityEngine.Event evt, UnityEngine.Matrix4x4 parentTransform, UnityEngine.Rect clippingRect, System.Boolean isComputingLayout, UnityEngine.Rect layoutSize, System.Action onGUIHandler, System.Boolean canAffectFocus) (at /Users/bokken/build/output/unity/unity/Modules/UIElements/Core/IMGUIContainer.cs:407)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&) (at /Users/bokken/build/output/unity/unity/Modules/IMGUI/GUIUtility.cs:224)