-MissingReferenceException errors are thrown continuously on assigning Orbital Transposer for Body property in Virtual Camera prefab.
-Body and Aim properties disappear after assigning Orbital Transposer for Body.

Steps to repro:
1. Create a new Project.
2. Windows > Package Manager > Cinemachine > Install.
3. Create a Virtual Camera from Cinemachine menu.
4. Create prefab of Virtual Camera(CM vcam1).
5. Assign "Orbital Transposer" to Body property.

Actual Result:
MissingReferenceException errors are thrown in the console.

Expected Result:
No errors in the console.

Reproducible in:
2019.3.0f6 (4f179a04ad78), 2019.3.0a3.

Working Fine in:
2020.1.0a20, 2019.2.1f1.

Note: Could not Test on 2019.3.0a2, 2019.3.0a1 due to editor freezes on installing Cinemachine package.

Tested with Package Version: 2.5.0, 2.4.0, 2.3.4, 2.3.3.

Environment:
Occurring on Windows & Mac.

Stack trace:
MissingReferenceException: The object of type 'CinemachineComposer' has been destroyed but you are still trying to access it.
Your script should either check if it is null or you should not destroy the object.
Cinemachine.CinemachineComposer.get_IsValid () (at library/packagecache/com.unity.cinemachine@2.3.4/runtime/components/cinemachinecomposer.cs:114)
Cinemachine.Editor.CinemachineVirtualCameraEditor.UpdateStageState (Cinemachine.CinemachineComponentBase[] components) (at library/packagecache/com.unity.cinemachine@2.3.4/editor/editors/cinemachinevirtualcameraeditor.cs:387)
Cinemachine.Editor.CinemachineVirtualCameraEditor.UpdateInstanceData () (at library/packagecache/com.unity.cinemachine@2.3.4/editor/editors/cinemachinevirtualcameraeditor.cs:336)
Cinemachine.Editor.CinemachineVirtualCameraEditor.DrawPipelineInInspector () (at library/packagecache/com.unity.cinemachine@2.3.4/editor/editors/cinemachinevirtualcameraeditor.cs:144)
Cinemachine.Editor.CinemachineVirtualCameraEditor.OnInspectorGUI () (at library/packagecache/com.unity.cinemachine@2.3.4/editor/editors/cinemachinevirtualcameraeditor.cs:105)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass55_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <34845f229fdb47a1a24a5737f3a3357b>:0)
UnityEditor.PopupCallbackInfo:SetEnumValueDelegate(Object, String[], Int32)