NullReferenceException is thrown and properties disappear on enabling "Avoid Obstacle checkbox" in CinemachineCollider preset asset.

Steps to repro:
1. Create a new Project.
2. Windows > Package Manager > Install Cinemachine package 2.3.4.
3. Cinemachine > Create Virtual Camera > Add Extension to Cinemachine Collider.
4. Create CinemachineCollider preset asset.
5. Enable "Avoid Obstacle checkbox" in CinemachineCollider preset asset.

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.Editor.CinemachineColliderEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.cinemachine@2.3.3/Editor/Editors/CinemachineColliderEditor.cs:34)
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)