Steps to reproduce:
1. Create a new project using the Universal 3D template
2. Install Multiplayer Play Mode package via Package Manager window (or add package by name: com.unity.multiplayer.playmode)
3. Add the "LaunchingScenarioWindowRepro.cs" provided script to the Assets folder (or just Project window)
4. Select QA > Repro > Open LaunchingScenarioWindow from the main menu
5. Observe the Console window and the opened window

Actual results: an unhandled NullReferenceException is thrown from LaunchingScenarioWindow.CreateGUI and the window renders nothing. The script logs "rootVisualElement child count = 0"

Expected results: the window either renders its UI, or does not open and explains why, no errors are thrown in the Console window

Reproducible with versions: 2.0.2 (6000.4.0a1, 6000.5.11f1), 3.0.0 (6000.6.1f1, 6000.7.0a6)

Not reproducible with versions: 2.0.2 (6000.3.0a6, 6000.3.24f1)

Tested on (OS): macOS 26.6.1 (Apple silicon)

Notes: Editor regression, not package

NullReferenceException: Object reference not set to an instance of an object
Unity.Multiplayer.PlayMode.Editor.LaunchingScenarioWindow.CreateGUI () (at /Users/bokken/build/output/unity/unity/Modules/MultiplayerEditor/Managed/MultiplayerPlayMode/PlayModeConfig/UI/LaunchingScenarioWindow.cs:168)
System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <0e19fd8bd8e94908bba2ee4f803bc6a7>:0)
Rethrow as TargetInvocationException: Exception has been thrown by the target of an invocation.
System.Reflection.RuntimeMethodInfo.Invoke (System.Object obj, System.Reflection.BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) (at <0e19fd8bd8e94908bba2ee4f803bc6a7>:0)
System.Reflection.MethodBase.Invoke (System.Object obj, System.Object[] parameters) (at <0e19fd8bd8e94908bba2ee4f803bc6a7>:0)
UnityEditor.UIElements.DefaultEditorWindowBackend.Invoke (System.String methodName) (at /Users/bokken/build/output/unity/unity/Modules/UIElementsEditor/WindowBackends/DefaultEditorWindowBackend.cs:364)
UnityEditor.EditorWindow:GetWindow(Type, Boolean, String, Boolean)
LaunchingScenarioWindowRepro:Open() (at Assets/LaunchingScenarioWindowRepro.cs:20)