NullReferenceException error is thrown on selecting CinemachineFreeLook camera preset asset.
It also throws error on changing any values in preset asset.

Steps to repro:
1. Create a new Project.
2. Windows > Package Manager > Install Cinemachine package 2.3.4.
3. Cinemachine > Create FreeLook Camera.
4. Create CinemachineFreeLookpreset asset.
5. Select CinemachineFreeLookpreset asset in the project view.

Actual Result:
NullReferenceException error is thrown in the console.

Expected Result:
No error in the console.

Reproducible in Package Version: 2.3.4, 2.3.3.

Tested in Unity Version:
2020.1.0a9, 2019.3.0b7, 2019.2.0f1.

Environment:
Windows 10 & Mac 10.14.

Stack trace:
NullReferenceException: Object reference not set to an instance of an object
Cinemachine.CinemachineFreeLook.UpdateRigCache () (at library/packagecache/com.unity.cinemachine@2.3.4/runtime/behaviours/cinemachinefreelook.cs:546)
Cinemachine.CinemachineFreeLook.GetRig (System.Int32 i) (at library/packagecache/com.unity.cinemachine@2.3.4/runtime/behaviours/cinemachinefreelook.cs:157)
Cinemachine.CinemachineFreeLookEditor.UpdateRigEditors () (at library/packagecache/com.unity.cinemachine@2.3.4/editor/editors/cinemachinefreelookeditor.cs:97)
Cinemachine.CinemachineFreeLookEditor.OnInspectorGUI () (at library/packagecache/com.unity.cinemachine@2.3.4/editor/editors/cinemachinefreelookeditor.cs:67)
UnityEditor.Presets.PresetEditor.DrawInternalInspector () (at <2a08bc3d8f9c4796ba99475f6f6c6a6b>:0)
UnityEditor.Presets.PresetEditor.DrawPresetData () (at <2a08bc3d8f9c4796ba99475f6f6c6a6b>:0)
UnityEditor.Presets.PresetEditor.OnInspectorGUI () (at <2a08bc3d8f9c4796ba99475f6f6c6a6b>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass54_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <2a08bc3d8f9c4796ba99475f6f6c6a6b>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)