TinyMCE cannot be rendered within CollectionEditorDescriptor

Description

Steps to reproduce
1. Create a block that has a Property<T> with an XHtmlString property

[ContentType(GUID = "cb788228-13ce-4cae-aee3-9e08261ccc8f")]
    public class MyBlock : SiteBlockData
    {
        [CultureSpecific]
        [Display(Order = 100, GroupName = SystemTabNames.Content)]
        public virtual string Heading { get; set; }
 
        [CultureSpecific]
        [EditorDescriptor(EditorDescriptorType = typeof(CollectionEditorDescriptor<ValueItem>))]
        [Display(Order = 200, GroupName = SystemTabNames.Content)]
        public virtual IList<ValueItem> Items { get; set; }
 
    }
 
    [PropertyDefinitionTypePlugIn]
    public class AppendixTypeProperty : PropertyList<ValueItem>
    {
        public AppendixTypeProperty()
        {
            _objectSerializer = this._objectSerializerFactory.Service.GetSerializer("application/json");
        }
 
        private Injected<ObjectSerializerFactory> _objectSerializerFactory;
        private IObjectSerializer _objectSerializer;
 
        protected override ValueItem ParseItem(string value)
        {
            return _objectSerializer.Deserialize<ValueItem>(value);
        }
    }
 
    public class ValueItem
    {
        public string Text { get; set; }
 
        public XhtmlString Body { get; set; }
    }

2. Create the block
3. Open "All Properties" View

Expected: there should have a plus button allowing editors to open add a row
Actual: The button cannot be shown and in console, there is a message saying internal server error.