Steps to reproduce:

1. Open the attached project
2. Open the SampleScene
3. Select the Terrain object in the Hierarchy
4. In the Inspector window, select the Cylinder in Trees section
5. Click the Edit Trees button and select the Edit Tree option
6. Edit Tree window opens as expected
7. Open the New Scene in the Project window

Actual results: MissingReferenceException and GUI Errors are spammed in the Console

Expected results: Edit Tree window closes or prompts the user to select a Terrain object in order to proceed

Reproducible with versions: 6000.0.61f1, 6000.2.12f1, 6000.3.0b9, 6000.4.0a4

Tested on (OS): macOS 26.0.1

Notes:

• The issue also occurs if the Edit Tree window was opened while working with Terrain in Prefab mode and the user exits Prefab mode