Problem with propertygrid

using System;
using System.ComponentModel;
using System.Drawing.Design;
using System.Windows.Forms;
using System.Windows.Forms.Design;

namespace PropertyGridTest
{
    static class Program
    {
        class TestPropertyEditor : UITypeEditor
        {
            public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context)
            {
                return UITypeEditorEditStyle.Modal;
            }

            public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
            {
                IWindowsFormsEditorService editorservice = provider.GetService(typeof(IWindowsFormsEditorService)) as IWindowsFormsEditorService;
                if (editorservice != null)
                {
                    // Let's create a test dialog to select some strings
                    // - Replace this whith your selection dialog form and your objects
                    Form formDialog = new Form();
                    Button buttonOk = new Button { Text = "Ok", Dock = DockStyle.Bottom, DialogResult = System.Windows.Forms.DialogResult.OK, Parent = formDialog };
                    Button buttonCancel = new Button { Text = "Cancel", Dock = DockStyle.Bottom, DialogResult = System.Windows.Forms.DialogResult.Cancel, Parent = formDialog };
                    formDialog.CancelButton = buttonCancel;
                    formDialog.AcceptButton = buttonOk;
                    ListBox listbox = new ListBox { Dock = DockStyle.Fill, Parent = formDialog };
                    // Add some test items to select
                    listbox.Items.AddRange(new object[] { "Test1", "Test2", "Test3" });

                    // re-select current value
                    if (value != null)
                        listbox.SelectedItem = value;

                    // show the dialog
                    if (editorservice.ShowDialog(formDialog) == DialogResult.OK)
                    {
                        // set the selected object as new value (which is returned)
                        value = listbox.SelectedItem;
                    }

                }
                return value;
            }
        }

        class TestObject
        {
            // The Property with the special editor
            [Editor(typeof(TestPropertyEditor), typeof(UITypeEditor))]
            public string TestProperty { get; set; }
        }

        static void Main()
        {
            // Create a simple test Form with a PropertyGrid
            Form formMain = new Form();
            PropertyGrid pg = new PropertyGrid { Dock = DockStyle.Fill, Parent = formMain };
            pg.SelectedObject = new TestObject();

            Application.Run(formMain);
        }
    }
}