ArgumentNullException error is thrown on selecting CinemachineBlendListCamera preset asset after deleting BlendList Camera in Hierarchy.

Steps to repro:
1. Create a new project.
2. Windows > Package Manager > Install Cinemachine package 2.3.4.
3. Cinemachine > Create BlendList Camera.
4. Create CinemachineBlendListCamera preset asset.
5. Delete BlendList Camera in Hierarchy.
6. Select CinemachineBlendListCamera 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:
ArgumentNullException: Value cannot be null.
Parameter name: key
System.Collections.Generic.Dictionary`2[TKey,TValue].TryInsert (TKey key, TValue value, System.Collections.Generic.InsertionBehavior behavior) (at <437ba245d8404784b9fbab9b439ac908>:0)
System.Collections.Generic.Dictionary`2[TKey,TValue].set_Item (TKey key, TValue value) (at <437ba245d8404784b9fbab9b439ac908>:0)
Cinemachine.Editor.CinemachineBlendListCameraEditor.UpdateCameraCandidates () (at Library/PackageCache/com.unity.cinemachine@2.3.3/Editor/Editors/CinemachineBlendListCameraEditor.cs:76)
Cinemachine.Editor.CinemachineBlendListCameraEditor.OnInspectorGUI () (at Library/PackageCache/com.unity.cinemachine@2.3.3/Editor/Editors/CinemachineBlendListCameraEditor.cs:48)
UnityEditor.Presets.PresetEditor.DrawInternalInspector () (at <9f61046f43c743d99486c2fccce7b774>:0)
UnityEditor.Presets.PresetEditor.DrawPresetData () (at <9f61046f43c743d99486c2fccce7b774>:0)
UnityEditor.Presets.PresetEditor.OnInspectorGUI () (at <9f61046f43c743d99486c2fccce7b774>:0)
UnityEditor.UIElements.InspectorElement+<>c__DisplayClass54_0.<CreateIMGUIInspectorFromEditor>b__0 () (at <9f61046f43c743d99486c2fccce7b774>:0)
UnityEngine.GUIUtility:ProcessEvent(Int32, IntPtr)