Datatype dropdown in XAML designer not showing custom Class

Question

I have a TreeView with HierarchicalDataTemplate that I'm trying to bind with an ObservableCollection of custom types.
But the HierarchicalDataTemplate's DataType attribute's dropdown list of available types in my Namespace is incomplete, it's missing the TFolderItem custom type, but listing all the other custom types in that same namespace. The namespace is MyProject.Classes, and classes are in plain Classes folder in the project directory.
I don't understand why it's not showing in the XAML code editor dropdown.

public class TFolderItem
{
    /*public FolderItem(RemoteDirectoryInfo rdi, WinSCP.Session winscpSession)
    {
        RDI = rdi;
        this.WinSCPSession = winscpSession;
    }*/

    public TFolderItem(string path, WinSCP.Session winscpSession)
    {
        RDI = winscpSession.ListDirectory(path);
        this.FtpPath = path;
        this.WinSCPSession = winscpSession;
    }

    private WinSCP.Session winscpSession;

    public RemoteDirectoryInfo RDI { get; set; }

    public string FtpPath { get; set; }

    public WinSCP.Session WinSCPSession
    {
        get { return this.winscpSession; }
        set { this.winscpSession = value; }
    }

    public IList Children
    {
        get
        {
            var children = new CompositeCollection();

            var subDirItems = new List<TFolderItem>();
            var subDirFiles = new List<RemoteFileInfo>();

            foreach (RemoteFileInfo rfi in RDI.Files)
            {
                if (rfi.IsDirectory)
                {
                    subDirItems.Add(new TFolderItem(this.FtpPath + rfi.Name + "/", this.WinSCPSession));
                }
                else
                {
                    subDirFiles.Add(rfi);
                }
            }

            children.Add(new CollectionContainer
            {
                Collection = subDirItems
            });
            children.Add(new CollectionContainer
            {
                Collection = subDirFiles
            });

            return Children;
        }
    }
}

Here is the view's xaml:

<UserControl x:Class="MyProject2.Views.FTPTab"
         xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
         xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
         xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
         xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
         xmlns:local="clr-namespace:MyProject2.Views"             
         xmlns:MyProject2Classes="clr-namespace:MyProject2.Classes"                 


         mc:Ignorable="d" 
         d:DesignHeight="300" d:DesignWidth="300">
<Grid>
    <TreeView  ItemsSource="{Binding FolderItems}" Height="300" Width="300">
        <TreeView.Resources >
            <HierarchicalDataTemplate DataType="" ItemsSource="{Binding Childrenx}">
                <TextBlock Text="{Binding FtpPathr}"/>
            </HierarchicalDataTemplate>
            <DataTemplate DataType=":">
                <TextBlock Text="{Binding Name}" />
            </DataTemplate>
        </TreeView.Resources>
    </TreeView>
</Grid>

This is the viewmodel:

public class FTPTabViewModel : BindableBase
{
    public FTPTabViewModel(string host, WinSCP.Session winscpSession)
    {
        this.Host = host;
        this.FolderItems = new ObservableCollection<TFolderItem>();
        this.Session = winscpSession;            

        this.FolderItems.Add(new TFolderItem("/",Session));
    }

    private WinSCP.Session session;
    private ObservableCollection<TFolderItem> folderItems;
    private string host;

    public string Host
    {
        get { return this.host; }
        set { this.host = value; }
    }

    public WinSCP.Session Session
    {
        get { return session; }
        set { this.session = value; }
    }

    public ObservableCollection<TFolderItem> FolderItems
    {
        get { return folderItems; }
        set { SetProperty(ref this.folderItems, value); }
    }
}

Answer 1

It seems that x:type dropdown only displays classes with default constructor. Adding one in TFolderItem class made it display in x:type dropdown.