'{DependencyProperty.UnsetValue}' is not a valid value for property 'FocusVisualStyle'

I have a weird error I'm trying to debug with no luck.

I have subclassed hwndhost showing some content, I have the following function in that class to set to fullscreen:

    private void SetFullScreen(bool enable)
    {
        if (enable)
        {
            fs = new Window();
            fs.ResizeMode = ResizeMode.NoResize;
            fs.WindowState = System.Windows.WindowState.Maximized;
            fs.WindowStyle = System.Windows.WindowStyle.None;
            fs.Topmost = true;
            fs.PreviewKeyDown += delegate(object sender, KeyEventArgs e) { 
                if (e.Key==Key.Escape)
                    FullScreen = false;
            };
            fs.Show();
        }
        else
        {
            fs.Close();
            fs = null;
        }
    }

This worked fine in my prototype WPF app but when I use this code in my main app I get this error when closing the window (escape key) and on fs.close() call:

'{DependencyProperty.UnsetValue}' is not a valid value for property 'FocusVisualStyle'.

The weird thing is it happens about 1500ms AFTER the window closes. I've tried setting FocusVisualStyle on fs to null, but it looks like something else. Gut feeling is it's trying to focus another element in my app that doesn't have this property, but really I have no idea!

Thanks!

Edit. Problem was custom setting of FocusVisualStyle on my fullscreen button. I set to {x:Null} and the problem went away.

my guess is that the control that gets the focus when you close the mentioned window has a custom style set by you that does not include any FocusVisualStyle.

so to help you further, you should explain a bit more: what happens (or should happen) when you close this window?

what control type is supposed to get the focus?