NullReferenceException error is thrown on adding "Hierarchy to Attribute Map" Property Binder.

Steps to repro:
1. Create a new HDRP project.
2. Hierarchy > Click on "+" > Visual Effects > Visual Effect
3. Inspector > Add Component > Audio Spectrum Binder.
4. Property Binding > Add > Point Cache > Hierarchy to Attribute Map.

Actual Result:
NullReferenceException error in the console.

Expected Result:
No error in the console.

Reproducible in:
2020.2.0b2, 2020.1.5f1, 2020.1.0b15 with Package Version 10.0.0-preview.27, 9.0.0-preview.33, 8.2.0.

Environment:
Occurring on Windows 10 & Mac 10.15.

Stacktrace:
NullReferenceException: Object reference not set to an instance of an object
UnityEngine.VFX.Utility.VFXHierarchyAttributeMapBinder.ChildrenOf (UnityEngine.Transform source, System.UInt32 depth) (at library/packagecache/com.unity.visualeffectgraph@10.0.0-preview.27/runtime/utilities/propertybinding/implementation/vfxhierarchyattributemapbinder.cs:72)
UnityEngine.VFX.Utility.VFXHierarchyAttributeMapBinder.UpdateHierarchy () (at library/packagecache/com.unity.visualeffectgraph@10.0.0-preview.27/runtime/utilities/propertybinding/implementation/vfxhierarchyattributemapbinder.cs:59)
UnityEngine.VFX.Utility.VFXHierarchyAttributeMapBinder.OnEnable () (at library/packagecache/com.unity.visualeffectgraph@10.0.0-preview.27/runtime/utilities/propertybinding/implementation/vfxhierarchyattributemapbinder.cs:49)
UnityEditor.GenericMenu:CatchMenu(Object, String[], Int32)