collection was modified enumeration operation may not execute.

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                TableRow tr = new TableRow();
            }
            else
            {
                if (Session["tblSystem"] != null)
                {
                    Table tb = (Table)Session["tblSystem"];
                    int count = tb.Rows.Count;
                    foreach (TableRow tr in tb.Rows)
                    {
                        tblSystem.Rows.Add(tr);
                    }
                }
            }
        }
        protected void CreateTextBox(object sender, ImageClickEventArgs e)
        {
            TextBox MyTextBox = new TextBox();
            //Assigning the textbox ID name 
            ImageButton img = sender as ImageButton;
            TableRow row = new TableRow();

            string button = img.AlternateText;
            switch (button)
            {
                case "System":
                    TableCell cell = new TableCell();
                    TableCell cell1 = new TableCell();
                    ImageButton minus = new ImageButton();

                    cell.ID = "tblCell" + ++system;
                    cell1.ID = "tblCell1" + system;
                    row.ID = "tblRow" + system;

                    minus.ID = "imgSystem" + system;
                    minus.ImageUrl = "~/Images/minus.png";

                    MyTextBox.ID = "txtSystem" + system;
                    MyTextBox.Width = 145;
                    MyTextBox.Height = 17;
                    MyTextBox.TextMode = TextBoxMode.SingleLine;

                    cell.Controls.Add(MyTextBox);
                    cell1.Controls.Add(minus);
                    //cell.Text = "Iteration : " + system.ToString();
                    //cell1.Text = "Iteration : " + system.ToString();
                    row.Cells.Add(cell);
                    row.Cells.Add(cell1);
                    tblSystem.Rows.Add(row);
                    Session["tblSystem"] = tblSystem;
                    minus.Click += new System.Web.UI.ImageClickEventHandler(deleteRow);
                    //pnlSystem.Controls.Add(MyTextBox);
                    break;
            }
        }
        protected void deleteRow(object sender, System.Web.UI.ImageClickEventArgs e)
        {
            TableRow row = new TableRow();
            ImageButton img = sender as ImageButton;
            string val = img.ID;
            string id = val.Substring(val.Length - 1, 1);
            if (val.Contains("System"))
            {
                TableRow rw = (TableRow)row.FindControl("tblRow" + id);
                rw.Visible = false;
            }
            Session["tblSystem"] = tblSystem;
        }

I am getting error in foreach in load event. Please help.
Thanks in advance