"Lifecycle ERROR : Exception ..." error is thrown when activating an Additional Editor Instance 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: "Lifecycle ERROR : Exception while executing scope transition handler Unity.Multiplayer.PlayMode.Editor.EditorModesUtility.InitializeOnLoad(): System.NullReferenceException ..." error is thrown

Expected results:  No errors are present in Player 2 instance

Reproducible with versions: 6000.7.0a5

Not reproducible with versions: 6000.3.21f1, 6000.5.8f1, 6000.6.0b9, 6000.7.0a4

Tested on (OS): Windows 11, macOS 26.5

Notes:

• Before the Lifecycle ERROR in this ticket, other warnings and errors were present (reported in [Internal link]). But the error reported in this ticket only appeared in 6000.7.0a5