Steps to reproduce:

1. Create a new project using Mixed Reality template
2. Click Fix in the Project Validation Window
3. Observe the Console

Actual results: "InvalidOperationException: Cannot modify VisualElement hierarchy during layout calculation" and "NullReferenceException: Object reference not set to an instance of an object" errors are thrown
Expected results: No errors or warnings are thrown

Reproducible with versions: 6000.2.0a4
Not reproducible with versions: 6000.0.39f1, 6000.1.0b7, 6000.2.0a3
Can't test with versions: 2021.3.49f1 (No Mixed Reality template present)

Tested on (OS): macOS Sequoia 15.3 (M1), macOS Sequoia 15.2 (Intel)

Notes:

• Issue is reproducible on macOS Sequoia 15.2 (Intel) only after "Fix All" button is pressed on Project Validation window
• Full errors:

{code:java}
NullReferenceException: Object reference not set to an instance of an object Unity.XR.CoreUtils.Editor.ProjectValidationSettingsProvider.OnInspectorUpdate () (at ./Library/PackageCache/com.unity.xr.core-utils@c0df0dd5bced/Editor/ProjectValidation/ProjectValidationSettingsProvider.cs:45) UnityEditor.SettingsWindow.OnInspectorUpdate () (at /Users/bokken/build/output/unity/unity/Editor/Mono/Settings/SettingsWindow.cs:185) UnityEditor.HostView.OnInspectorUpdate () (at /Users/bokken/build/output/unity/unity/Editor/Mono/HostView.cs:452)
{code}