Problem in creating MDI window forms

I have been try to create a window form in which when we open a new child form it checks if the child form is already open or not. If not open it create an new form but if a child form already exits it brings that form to front. I have tried using this code. It works fine for the first time but when i close the child form once & try to open it again it generates an exception saying, "Cannot use a disposed object.".

Pls someone help..

Thanks in advance...

Code:

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

namespace TestProject
{
  public partial class Form1 : Form
  {
  public static int flag1 = 0;  // to check if form2 is already open or not...
  public static int flag2 = 0;  // to check if form3 is already open or not...
 
 Form2 newform = new Form2();
 Form3 newform1 = new Form3();

  public Form1()
  {
  InitializeComponent();
  }

  private void newToolStripMenuItem_Click(object sender, EventArgs e)
  {
  if (flag == 0)
  {
  flag = 1;
  newform.MdiParent = this;
  newform.Show();
  }
  else
  {
  newform.BringToFront();

  }
  }
 private void new2ToolStripMenuItem_Click(object sender, EventArgs e)
  {
  if (flag2 == 0)
  {
  flag2 = 1;
  newform1.MdiParent = this;
  newform1.Show();
  }
  else
  {
  newform1.BringToFront();

  }
  }
  }
}