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;
#
using UnityEngine.UIElements;
#
 
#
public class TestElement : VisualElement
#
{
#
    public [new|http://www.google.com/search?q=new+msdn.microsoft.com] class UxmlFactory : UxmlFactory<TestElement, UxmlTraits> \{ }
#
 
#
    public TestElement()
#
    {
#
        VisualTreeAsset uxml = AssetDatabase.LoadAssetAtPath<VisualTreeAsset>("Assets/Editor/UXML/TestElement.uxml");
#
        TemplateContainer root = uxml!.Instantiate();
#
        this.hierarchy.Add(root);
#
    }
#
}

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