Error after Adding WPF on VB.Net

[k3nnt0ter0]

I embedded the WPF to vb.net and I got some error.
1. There's still a green underline on line

Me.SampleUserControl1 = New SampleWpfUserControlLibrary.SampleUserControl()

but that's nothing I guess since the underline was green.
2. My very problem is on the line

_hostWindow.OpenDocument();

says

Object reference not set to an instance of an object.

My guess is that, it was suppose to be hosted on a Window but in my project's case, it was hosted on an Elementhost

Here's the code :

using System;
using System.Windows;
using System.Windows.Input;
using System.Windows.Media;

namespace SampleWpfUserControlLibrary
{
    /// <summary>
    /// Interaction logic for SampleUserControl.xaml
    /// </summary>
    public partial class SampleUserControl
    {
        private IApplicationHostWindow _hostWindow;
        
        [Obsolete("This constructor should not be used", false)]
        public SampleUserControl()
        {
            InitializeComponent();
        }

        public SampleUserControl(IApplicationHostWindow applicationHostWindow)
        {
            _hostWindow = applicationHostWindow;

            IsHitTestVisible = true;

            InitializeComponent();
        }

        public double GetViewboxWidth()
        {
            return viewHost.ActualWidth;
        }

        public double GetViewboxHeight()
        {
            return viewHost.ActualHeight;
        }

        public void SetViewboxChild(UIElement child)
        {
            viewHost.Content = child;
        }

        public void ProcessLeftButtonDown(Point point)
        {
            var result = VisualTreeHelper.HitTest(this, point);
            return;
        }

        protected override HitTestResult HitTestCore(PointHitTestParameters hitTestParameters)
        {
            return base.HitTestCore(hitTestParameters) ??
                   new PointHitTestResult(viewHost.Content as Visual ?? viewHost, hitTestParameters.HitPoint);
        }

        private void exitMenuItem_Click(object sender, RoutedEventArgs e)
        {
            _hostWindow.Exit();
        }

        private void newProjectMenuItem_Click(object sender, RoutedEventArgs e)
        {
             _hostWindow.OpenDocument();
        }

        private void UserControl_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            return;
        }

        private void UserControl_PreviewMouseLeftButtonDown(object sender, MouseButtonEventArgs e)
        {
            return;
        }
    }
}

It works fine when I run the WPF only. . But when I add it on the VB Project, error occurs