C# WindowsForm MVP - open double form when calling ShowDialog

Question

I'm learning the MVP pattern in C# to create simply Windows Forms app. I try recreate and testing this code in different situation and I found the problem with open double form when using method ShowDialog (when I using method Show everything works fine). Maybe someone can help me explain how to resolve this problem or tell me whats I did wrong (maybe I don't understand something or maybe this code is wrong).

App where simple and has 3 form

1. Main - with one button to open 2nd form and isMdiContainer is true

public partial class MainView : Form, IMainView
    {
        public MainView()
        {
            InitializeComponent();
            btnNewForm.Click += delegate { ShowNextView?.Invoke(this, EventArgs.Empty); };    
        }
        public event EventHandler ShowNextView;
    }

2. Second form - open in MdiContainer with button to open 3rd form

public partial class MyNewFormView : Form, IMyNewForm
    {
        //Costructor
        public MyNewFormView()
        {
            InitializeComponent();

            btnOpenShowDialog.Click += delegate { OpenEvent?.Invoke(this, EventArgs.Empty); };

        }
        public event EventHandler OpenEvent;

        //Static
        private static MyNewFormView instance;
        public static MyNewFormView GetInstance(Form parentContainer)
        {
            if (instance == null || instance.IsDisposed)
            {
                instance = new MyNewFormView();
                instance.MdiParent = parentContainer;
                instance.FormBorderStyle = FormBorderStyle.None;
                instance.Dock = DockStyle.Fill;
                instance.BringToFront();
            }
            else
            {
                if (instance.WindowState == FormWindowState.Minimized)
                    instance.WindowState = FormWindowState.Normal;
                instance.BringToFront();
            }
            return instance;
        }

    }

3. Last form - with two buttons OK and Cancel

    public partial class AddonView : Form, IAddonView
    {
        public AddonView()
        {
            InitializeComponent();
            btnOk.Click += delegate { Ok?.Invoke(this, EventArgs.Empty); };
            btnCancel.Click += delegate { Cancel?.Invoke(this, EventArgs.Empty); };
        }
        public event EventHandler Ok;
        public event EventHandler Cancel;

        private static AddonView instance;
        public static AddonView GetInstance(Form parentContainer)
        {
            if (instance == null || instance.IsDisposed)
            {
                instance = new AddonView();
                instance.FormBorderStyle = FormBorderStyle.None;
                instance.Dock = DockStyle.Fill;
                instance.BringToFront();
            }
            else
            {
                if (instance.WindowState == FormWindowState.Minimized)
                    instance.WindowState = FormWindowState.Normal;
                instance.BringToFront();
            }
            return instance;
        }
    }

This 3 forms runs from 3 class presenter

1. Main

   public class MainPresenter
    {
        private IMainView mainView;
        public MainPresenter(IMainView mainView)
        {
            this.mainView = mainView;
            this.mainView.ShowNextView += ShowNextView;
            this.mainView.Show();
        }
        private void ShowNextView(object sender, EventArgs e)
        {
            IMyNewForm view = MyNewFormView.GetInstance((MainView)this.mainView);
            new MyNewFormPresenter(view);
        }
    }

2. Second

 public class MyNewFormPresenter
    {
        IMyNewForm view;
        public MyNewFormPresenter(IMyNewForm view)
        {
            this.view = view;
            this.view.OpenEvent += OpenEvent;
            this.view.Show();
        }
        private void OpenEvent(object sender, EventArgs e)
        {
            IAddonView view = AddonView.GetInstance((MyNewFormView)this.view);
            new AddonPresenter(view);
        }
    }

3. The last one

   class AddonPresenter
    {
        private IAddonView addonView;

        public AddonPresenter(IAddonView addonView)
        {
            this.addonView= addonView;
            this.addonView.Ok += Ok;
            this.addonView.Cancel += Cancel;
            this.addonView.ShowDialog();
        }
        private void Ok(object sender, EventArgs e)
        {
            this.addonView.Dispose();
        }
        private void Cancel(object sender, EventArgs e)
        {
            this.addonView.Dispose();
        }
    }

I have 3 interfaces but they have only events, Show, and ShowDialog methods.

So whats the problem ... When I run app open first form, then call second, then call third everything work fine. On last I can close form with OK or Cancel button, and then from second I can call last form and form open single.

But when I run first form, then call second and then calls one more time second (from first) then when I open the last one I have two form. When run one more time button on first form to open second on the last one form will be one 3 times.

When I try call methods SHOW everythings works fine - each form open only one times (but then user can call 3rd form many times) when I use ShowDialog 3rd form open one time (until user don't run on on main menu 2nd form). I read difference between Show and ShowDialog but I have to small experience to find solution to avoid this problem. Could anybody help me resolve this problem?