NullReferenceException when building custom element tree from uxml

Steps to repro:

1. Create a custom VisualElement named TestElement, that has a Label as a child element. Since I want to avoid building the visual tree in C#, my class contains the following:
  #
using UnityEditor;

1. 
   using UnityEngine.UIElements;

2.  

3. 
   public class TestElement : VisualElement
4. { #     public [new|http://www.google.com/search?q=new+msdn.microsoft.com] class UxmlFactory : UxmlFactory<TestElement, UxmlTraits> \{ }

5.  

6.     public TestElement()

7.     

   { #         VisualTreeAsset uxml = AssetDatabase.LoadAssetAtPath<VisualTreeAsset>("Assets/Editor/UXML/TestElement.uxml"); #         TemplateContainer root = uxml!.Instantiate(); #         this.hierarchy.Add(root); #     }

8. }

2. When selecting the Label element in the Hierarchy Panel of the UI Builder, you get a NullReferenceException and the Inspector Panel doesn't refresh all the Label infos correctly.

NullReferenceException: Object reference not set to an instance of an object
Unity.UI.Builder.BuilderInspectorAttributes.RefreshAttributeField (UnityEngine.UIElements.BindableElement fieldElement) (at <415ec9d438ed4f3badc4f75a27f3e7ff>:0)
Unity.UI.Builder.BuilderInspectorAttributes.CreateAttributeRow (UnityEngine.UIElements.UxmlAttributeDescription attribute) (at <415ec9d438ed4f3badc4f75a27f3e7ff>:0)
Unity.UI.Builder.BuilderInspectorAttributes.GenerateAttributeFields () (at <415ec9d438ed4f3badc4f75a27f3e7ff>:0)
Unity.UI.Builder.BuilderInspectorAttributes.Refresh () (at <415ec9d438ed4f3badc4f75a27f3e7ff>:0)
Unity.UI.Builder.BuilderInspector.RefreshUI () (at <415ec9d438ed4f3badc4f75a27f3e7ff>:0)
Unity.UI.Builder.BuilderInspector.SelectionChanged () (at <415ec9d438ed4f3badc4f75a27f3e7ff>:0)

 

The culprit seems to be:
 
internal void CloneTree(VisualElement target, Dictionary<string, VisualElement> slotInsertionPoints, List<TemplateAsset.AttributeOverride> attributeOverrides)
 
... gets called with the attributeOverrides set to null whenever Clone() or Instantiate() is called