Property as ContentArea inside PropertyList Item

Hello all,

I have a Block:

public class ExpandableBlock : BlockData
{
    [Required]
    [Display(Name = "Heading", GroupName = SystemTabNames.Content)]
    public virtual string Heading { get; set; }
    [Required]
    [UIHint(UIHint.Textarea)]
    [Display(Name = "Sub-Text", GroupName = SystemTabNames.Content)]
    public virtual string SubText { get; set; }
    [Required]
    [EditorDescriptor(EditorDescriptorType = typeof(CollectionEditorDescriptor<ExpandableBlockItem>))]
    public virtual IList<ExpandableBlockItem> Items { get; set; }
}

And ExpandableBlockItem Class:

public class ExpandableBlockItem
{
    [Required]
    [Display(Name = "Button Text")]
    public virtual string ButtonText { get; set; }
    [Required]
    [UIHint(UIHint.Block)]
    [Display(Name = "Expandable Area")]
    public virtual ContentArea ExpandableContent { get; set; }
    [Ignore]
    [ScaffoldColumn(false)]
    public virtual string Id => Guid.NewGuid().ToString();
}

Under every ExpandableBlockItem I want to have a ContentArea, so I could put some Block inside.

However it doesn't work and does not allow me to create new block under ExpandableContent property (this is how it renderds - https://i.imgur.com/2uPJCYvl.png). It renders as an area where I can drop some blocks with a text - You can drop content here. - but without a link to Create New Block or Media folder so I could drag some block there.

Thank you in advance, cheers!