Terrain’s “Add Tree” window does not disable the “Add” button when the Tree Prefab is deleted and missing, allowing a “Missing” Tree to be added

Steps to reproduce:

1. Create a new Unity project using the Universal 3D template
2. Create a new Terrain GameObject (GameObject > 3D Object > Terrain)
3. Create a new Tree GameObject (GameObject > 3D Object > Tree)
4. Select the Terrain GameObject in the Hierarchy window
5. Open the "Paint Trees" tab in the Terrain component
6. Click the "Edit Trees" button and select "Add Tree" option
7. Select the created Tree as the Tree Prefab in the "Add Tree" window
8. Delete the "Assets/Tree.prefab" prefab
9. Observe the "Add Tree" window
10. Click the "Add" button in the "Add Tree" window
11. Observe the Terrain component

Actual results: The "Add" button is not disabled when a Tree with a missing Prefab is selected

Expected results: The "Add" button is disabled or the Tree with a missing Prefab is not added

Reproducible with versions: 6000.0.65f1, 6000.3.4f1, 6000.4.0b3, 6000.5.0a4

Tested on (OS): MacOS Silicon Tahoe 26.1