System.ObjectDisposedException after changing from WPF project to WinForms context

Question

The application was started from an empty C# project, output type set to "Windows App":

using System.Windows.Forms;

static class Program
{
    static System.Windows.Forms.NotifyIcon notifyIcon;
    static System.Drawing.Icon LockedIcon = Properties.Resources.icon_locked;

    [STAThread]
    Main()
    {
        Application.EnableVisualStyles();
        Application.SetCompatibleTextRenderingDefault(false);
        notifyIcon = new NotifyIcon()
        {
            Icon = LockedIcon,
            ContextMenu = ContextualMenu(),
            Visible = true
        };
        Application.Run()
    }
}

I am using WinForms as a base since WPF doesn't have a systray icon control. However, all the other windows are WPF. I am running them like this:

static void ShowUI()
{
    MainUI wnd = new MainUI();  //MainUI is a System.Windows.Window (WPF)
    wnd.Show();
}

I have a reference to WindowsFormsIntegration. The WPF window shows correctly and everything seems to work. But:

public MainUI()
{
    this.Loaded += (o,e) => this.Icon = Properties.Resources.myIcon.ToImageSource();
    InitializeComponent();
}

public static System.Windows.Media.ImageSource ToImageSource(this System.Drawing.Icon i)
{
    System.Windows.Media.ImageSource imgsrc;
    using (System.IO.MemoryStream iconStream = new System.IO.MemoryStream())
    {
        i.Save(iconStream);
        iconStream.Seek(0, System.IO.SeekOrigin.Begin);
        imgsrc = System.Windows.Media.Imaging.BitmapFrame.Create(iconStream);
    }
    return imgsrc;
}

...throws a System.ObjectDisposedException. The extension method ToImageSource() worked fine when it was a pure WPF project, but ever since I switched to a WinForms context, it stopped working and throws that exception.

I tried debugging by splitting the one-liner into several steps:

System.Drawing.Icon icon = Properties.Resources.myIcon; //works
ImageSource imgsrc = icon.ToImageSource(); //works
wnd.Icon = imgsrc; //System.ObjectDisposedException

Not sure what's happening. Is it a side effect of not running the actual WPF Application.Run()?