Reproduction steps:
1. Create a new project
2. Install Multiplayer Play Mode package (Window > Package Management > Package Manager)
3. Open Play Mode Scenarios window (Window > Play Mode > Scenarios), add a new Scenario and an Additional Editor Instance
4. Select the newly created Scenario as the active Scenario and enter Play mode
5. Enable Hierarchy in the Player 2 window Layout dropdown
6. Create Tree GameObject in the Player 2 Hierarchy (right-click > 3D Object > Tree)
7. Observe the Console

Actual result: "ArgumentNullException: Value cannot be null. Parameter name: sameAssetFile" error is thrown
Expected result: No errors or warnings are thrown

Reproducible with: 1.6.3 (6000.0.80f1), 2.0.2 (6000.3.20f1, 6000.5.6f1), 3.0.0 (6000.6.0b6, 6000.7.0a3)

Reproduced on: Windows 11, macOS Tahoe 26.5 (M4)

Note:
Full error:

ArgumentNullException: Value cannot be null.
Parameter name: sameAssetFile
UnityEditor.AssetDatabase.AddObjectToAsset_Obj (UnityEngine.Object newAsset, UnityEngine.Object sameAssetFile) (at <b9860ef9ab5e43a4a09be932a4cd5645>:0)
UnityEditor.AssetDatabase.AddObjectToAsset (UnityEngine.Object objectToAdd, UnityEngine.Object assetObject) (at /Users/bokken/build/output/unity/unity/Modules/AssetDatabase/Editor/ScriptBindings/AssetDatabase.bindings.cs:387)
TreeEditor.TreeEditor.CreateNewTree (UnityEditor.MenuCommand menuCommand) (at /Users/bokken/build/output/unity/unity/Modules/TreeEditor/TreeEditor.cs:334)
UnityEditor.MenuUtils:MenuCallback(Object)
Unity.Hierarchy.Editor.<>c__DisplayClass0_0:<AppendFromGenericMenu>b__0(DropdownMenuAction) (at /Users/bokken/build/output/unity/unity/Modules/HierarchyEditor/Managed/DropdownMenuHelper.cs:23)
UnityEditor.GenericMenu:CatchMenu(Object, String[], Int32) (at /Users/bokken/build/output/unity/unity/Editor/Mono/GUI/GenericMenu.cs:164)