ERROR: A first chance exception of type 'System.InvalidCastException' occurred in WebKitBrowser.dll

Question

I am trying to open multiple URLs in my application in sequential order after few seconds of wait. But I am getting below exception while running my project. I am putting my code also here, please let me know what I am doing wrong and what is the best way to implement this requirement. Please let me know if you need any other information about my project configuration.

Exception: A first chance exception of type 'System.NotImplementedException' occurred in WebKitBrowser.dll
A first chance exception of type 'System.UriFormatException' occurred in System.dll
A first chance exception of type 'System.InvalidCastException' occurred in WebKitBrowser.dll
A first chance exception of type 'System.InvalidCastException' occurred in WebKitBrowser.dll
A first chance exception of type 'System.InvalidCastException' occurred in WebKitBrowser.dll
The thread 0x3b8c has exited with code 259 (0x103).
The thread 0xf04 has exited with code 259 (0x103).
The program '[0x2F30] WebKitProject.vshost.exe: Program Trace' has exited with code 0 (0x0).
The program '[0x2F30] WebKitProject.vshost.exe' has exited with code 0 (0x0).

Program:

    using System;
    using System.Collections.Generic;
    using System.ComponentModel;
    using System.Data;
    using System.Drawing;
    using System.Linq;
    using System.Text;
    using System.Threading;
    using System.Windows.Forms;
    using WebKit.Interop;

    namespace WebKitProject
    {
        public partial class Form1 : Form
        {                
            private string[] urls = new string[] { "http://www.google.com", "http://www.whatsmyip.org", "http://www.bling.com", "http://www.facebook.com" };
            int urlPointer = 0;


            public Form1()
            {
                InitializeComponent();            
            }

            private void Form1_Load(object sender, EventArgs e)
            {
               openBrowser(sender, e);           
            }

            private void openBrowser(object sender, EventArgs e)
            {
                if (urlPointer < urls.Length)
                {
                    String urlStr = urls[urlPointer];
                    urlPointer++;

                    this.webKitBrowser1.DocumentCompleted += new WebBrowserDocumentCompletedEventHandler(webKitBrowser1_Completed);
                    webKitBrowser1.Navigate(urlStr);
                }       
            }

            void webKitBrowser1_Completed(object sender, WebBrowserDocumentCompletedEventArgs e)
            {
                System.Timers.Timer timer = new System.Timers.Timer(5000);
                timer.Elapsed += new System.Timers.ElapsedEventHandler(openBrowser);
                timer.Start();
            }
        }
    }