Telerik Form running as Dialog stuck sometimes

Question

I'm using two RadTreeView to Drag and Drop Nodes from Right to Left. When "Node 1" is Drag from Right and Drop to Left RadTreeView a Dialog is shown.

If I Drag and Drop other Node before "Node 1" the Dialog stuck and I cannot write on the texboxes.

How can I solve this issue?

Main Form

public partial class Main : RadForm
{
    public Main()
    {
        InitializeComponent();
    }

    private void RightRadTreeView_DragEnded(object sender, RadTreeViewDragEventArgs e)
    {
        if (e.Node.Text == "Node 1")
        {
            new RadForm1().ShowDialog();
        }
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        //Right
        this.RightRadTreeView.Nodes.Add(new RadTreeNode { Text = "Node 1" });
        this.RightRadTreeView.Nodes.Add(new RadTreeNode { Text = "Node 2" });
        this.RightRadTreeView.Nodes.Add(new RadTreeNode { Text = "Node 3" });
        this.RightRadTreeView.Nodes.Add(new RadTreeNode { Text = "Node 4" });


    }

}

Dialog Form

   public partial class NewForm : Form
    {
        public NewForm()
        {
            InitializeComponent();

        }

        private void NewForm_Load(object sender, EventArgs e)
        {

        }
    }