ArcMap keeps crashing whilst using simple C# .NET addin

I honestly have no clue what is causing this.

I want to have a message appear that tells me which layers are selected or not.

Here is my code:

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;
using ESRI.ArcGIS.ArcMapUI;
using ESRI.ArcGIS.ArcMap;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.Framework;
using System.Windows.Forms;

namespace SelectedLayersTest2
{
    public class Button1 : ESRI.ArcGIS.Desktop.AddIns.Button
    {
        public Button1()
        {
        }

        protected override void OnClick()
        {

            IMxDocument mxdoc = ArcMap.Application.Document as IMxDocument;
            IMap map = mxdoc.FocusMap;

            ILayer layer;
            string message = "";

            for (int i = 0; i < map.LayerCount; i++)
            {
                layer = map.Layer[i];
                if (mxdoc.SelectedLayer == layer)
                {
                    message += layer.Name + "\n";
                }
                else
                {
                    message += layer.Name + " not selected\n";
                }
            }

            Message msgForm = new Message();
            msgForm.lbl.Text = message;
            msgForm.ShowDialog();

            ArcMap.Application.CurrentTool = null;
        }
        protected override void OnUpdate()
        {
            Enabled = ArcMap.Application != null;
        }
    }

}

This seems like such a harmless piece of code. What is causing this?