How to keep UITypeEditor visible in PropertyGrid when object is not selected?

Question

I'm writing a C# Windows form program and I'm having a hard time with PropertyGrids and UITypeEditor... I've read this post already: How to create custom PropertyGrid editor item which opens a form? which helped me getting started.

Here's the basic code I've got so far:

public FooClass Foo { get; private set; }

[Editor(typeof(FooEditor), typeof(UITypeEditor)),
TypeConverter(typeof(FooType))]
public class FooClass
{
    private bool _enabled;
    public bool Enabled
    {
        get { return _enabled; }
        set { _enabled = value; }
    }

    public void ShowWindow()
    {
        using (var test = new Form())
        {
            test.ShowDialog();    
        }
    }
}

private class FooType : ExpandableObjectConverter
{
    public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)
    {
        return "Click on the button to show the window";
    }
}

private class FooEditor : UITypeEditor
{
    public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context)
    {
        return UITypeEditorEditStyle.Modal;
    }

    public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value)
    {
        var svc = provider.GetService(typeof(IWindowsFormsEditorService)) as IWindowsFormsEditorService;
        var foo = value as FooClass;
        if (svc != null && foo != null)
        {
            foo.ShowWindow();
        }
        return null;
    }
}

That works as expected: when the Foo object is selected in the propertygrid I see the "..." button on the right, I click and the window shows up:

What I can't figure out is how to make this "..." button stay visible even when the Foo object is not selected in the propertygrid:

Dis I miss something simple?