How to reproduce:
1. Open the attached project "Test"
2. In the Project window, open the "Cube" Prefab
3. In the Hierarchy, select the "Capsule" GameObject
4. Make any change in the Inspector (e.g. on the Script Component)
5. In the Inspector, click "Overrides" under the Prefab field
6. Click on the changed Component to call the pop-up
7. In the "NetworkBehaviours require a NetworkObject" pop-up click "No (manually add it)" and observe the Console

Expected results: No errors are thrown
Actual results: "NullReferenceException" is thrown

Reproducible with: 6000.0.52f1, 6000.1.10f1, 6000.2.0b8, 6000.3.0a1

Reproducible on: macOS Sequoia 15.4 (M1)
Not reproducible on: No other environment tested

Note: Reproducible when clicking any option in the pop-up

First lines of error:
NullReferenceException: Object reference not set to an instance of an object
UnityEditor.EditorWindow.Close () (at /Users/bokken/build/output/unity/unity/Editor/Mono/EditorWindow.cs:1097)
UnityEditor.PopupWindowWithoutFocus.OnGlobalMouseOrKeyEvent (UnityEngine.EventType type, UnityEngine.KeyCode keyCode, UnityEngine.Vector2 mousePosition) (at /Users/bokken/build/output/unity/unity/Editor/Mono/GUI/PopupWindowWithoutFocus.cs:96)