How to reproduce:
1. Open the user's attached "Bug Test.zip" project
2. Open the "testscene" Scene
3. Select the "Main Camera" Prefab in the Project window
4. Add the "CinemachineVirtualCamera" Component in the Inspector
5. Observe the Console log

Expected result: No error messages are thrown in the Console log
Actual result: "Setting the parent of a transform which resides in a Prefab Asset is disabled to prevent data corruption (GameObject: 'cm')" error message is getting spammed in the Console log (Full error message in the edit)

Reproducible with: 2.6.1-preview.6, 2.6.1 (2018.4.26f1, 2019.4.9f1, 2020.1.4f1, 2020.2.0a21)
Not reproducible with: 2.6.0 (2018.4.26f1, 2019.4.9f1, 2020.1.4f1, 2020.2.0a21)

Note:
- If the Prefab's instance with a CinemachineVirtualCamera Component is selected in the Hierarchy and Scene view is opened, "NullReferenceException: Object reference not set to an instance of an object
Cinemachine.Editor.CinemachineVirtualCameraEditor.DrawVirtualCameraGizmos (Cinemachine.CinemachineVirtualCamera vcam, UnityEditor.GizmoType selectionType) (at Library/PackageCache/com.unity.cinemachine@2.6.1 preview.6/Editor/Editors/CinemachineVirtualCameraEditor.cs:553)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr, Boolean&)" error message is getting spammed in the Console log on 2020.1 stream