"FallbackEditorWindow" warning and "NullReferenceException" error is thrown when activating an Additional Editor Instance for the first time via custom Play Mode Scenario

Steps to reproduce:

1. Create new project
2. Install Multiplayer Play Mode package from the Package Manager (com.unity.multiplayer.playmode)
3. In the Toolbar, click on the "Default" dropdown next to Play mode icons
4. Click "Configure Play Mode Scenarios"
5. Click "+" to create new scenario
6. Click "+" under "Additional Editor Instances" to add Player 2
7. Click again on the "Default" dropdown and select "Activate Play mode Window"
8. Switch to the newly created scenario
9. Click "Activate" under Player 2
10. Observe the Console in Player 2 Editor instance

Actual results: "Attempted to open unsupported window in cloned player systems:" warnings and "NullReferenceException: Object reference not set to an instance of an object" error is thrown

Expected results: No errors or warnings are present in Player 2 instance

Reproducible with versions: 6000.5.6f1, 6000.6.0b6, 6000.7.0a3

Not reproducible with versions: 6000.3.21f1, 6000.5.5f1

Tested on (OS): macOS 26.5, Ubuntu 24.04

Notes:

• Only reproduces on the first activation of a given Player instance. Re-activating the same instance (e.g. stopping and starting Player 2 again) does not trigger the warning/error again. However, activating a new, previously unused instance (e.g. Player 3) reproduces it again on that instance's first activation