ArgumentException error is thrown on selecting CinemachineStateDrivenCamera preset asset after deleting StateDriven Camera in Hierarchy.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Install Cinemachine package 2.3.4.
3. Cinemachine > Create State-Driven Camera.
4. Create CinemachineStateDrivenCamera preset asset.
5. Delete State-Driven Camera in Hierarchy.
6. Select CinemachineStateDrivenCamera preset asset.

Actual Result:
ArgumentException error is thrown in the console.

Expected Result:
No error in the console.

Reproducible in Package Version: 2.3.4, 2.3.3.

Reproducible on:
2020.1.0a10, 2019.3.0b8, 2019.2.0f1.

Environment:
Windows 10 & Mac 10.14.

Stack trace:
ArgumentException: Object at index 0 is null
UnityEditor.SerializedObject..ctor (UnityEngine.Object obj) (at <6624fed2b51b48c3a769491bcd27ff2e>:0)
Cinemachine.Editor.CinemachineStateDrivenCameraEditor+<>c__DisplayClass20_0.<SetupChildList>b__2 (UnityEngine.Rect rect, System.Int32 index, System.Boolean isActive, System.Boolean isFocused) (at Library/PackageCache/com.unity.cinemachine@2.3.4/Editor/Editors/CinemachineStateDrivenCameraEditor.cs:417)
UnityEditorInternal.ReorderableList.DoListElements (UnityEngine.Rect listRect) (at <6624fed2b51b48c3a769491bcd27ff2e>:0)
UnityEditorInternal.ReorderableList.DoLayoutList () (at <6624fed2b51b48c3a769491bcd27ff2e>:0)
Cinemachine.Editor.CinemachineStateDrivenCameraEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.cinemachine@2.3.4/Editor/Editors/CinemachineStateDrivenCameraEditor.cs:98)
UnityEditor.Presets.PresetEditor.DrawInternalInspector () (at <6624fed2b51b48c3a769491bcd27ff2e>:0)
UnityEditor.Presets.PresetEditor.DrawPresetData () (at <6624fed2b51b48c3a769491bcd27ff2e>:0)
UnityEditor.Presets.PresetEditor.OnInspectorGUI () (at <6624fed2b51b48c3a769491bcd27ff2e>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass55_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <6624fed2b51b48c3a769491bcd27ff2e>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)