Object reference not set to an instance of an object due to the contentPage

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C#

ASP.NET

Hi, I have a masterpage and contentpages that inherit from it so in one of my contentPage I am putting this code:

<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <div>
        <asp:TextBox ID="txtNumber" runat="server">
        </asp:TextBox>
        <br />
        <asp:Button ID="btn" runat="server" Text="Generate" />
        <asp:Panel ID="pnlDate" runat="server">
        </asp:Panel>
    </div>
    <div>
        <asp:Button ID="btnSave" runat="server" Text="Save" OnClick="btn_Click" />
        <asp:Label ID="lblInsert" runat="server"></asp:Label>
    </div>
</asp:Content>

and the code behind is:

protected void Page_Load(object sender, EventArgs e)
    {
        GenerateTxtB();
    }
    protected void GenerateTextBoxes_onClick(object sender, EventArgs e)
    {
        pnlDate.Controls.Clear();
        GenerateTxtB();
    }
 
    private void GenerateTxtB()
    {
        if (txtNumber.Text != string.Empty)
        {
             int number = Convert.ToInt32(txtNumber.Text);
            string top = "";
            int left = 0;

            TextBox txt;
            Table tb = new Table();
            tb.ID = "tb1";
            TableRow tr = new TableRow();

            for (int i = 1; i <= number; i++)
            {
                Label lb = new Label();
                lb.ID = "lbFname" + i.ToString();
                lb.Text = "First Name :";

                txt = new TextBox();
                txt.ID = "textBox" + i.ToString();
                txt.Attributes.Add("runat", "server");

                txt.CssClass = "myDates";
                txt.Width = Unit.Pixel(240);
                txt.MaxLength = 1;
                txt.EnableViewState = true;
                txt.Style.Add(HtmlTextWriterStyle.Position, "relative");
                txt.Style[HtmlTextWriterStyle.Left] = left + "px";
                left += 5;


                RequiredFieldValidator rf = new RequiredFieldValidator();
                rf.ControlToValidate = "textBox" + i.ToString();
                rf.ID = "RF" + i.ToString();
                rf.Text = "Date are required";
                rf.ForeColor = System.Drawing.Color.Red;
                rf.ErrorMessage = "Date is required";
                rf.Display = ValidatorDisplay.Dynamic;
                rf.SetFocusOnError = true;
                pnlDate.Controls.Add(rf);


                TableCell tc = new TableCell();
                tc.Controls.Add(txt);
                tr.Controls.Add(tc);
                tb.Controls.Add(tr);
                pnlDate.Controls.Add(tb);

            }

        }

    }
    public void btn_Click(object sender, EventArgs e)
    {
        Table t = (Table)Page.FindControl("pnlDate").FindControl("tb1");
        TextBox textbox;

        foreach (TableRow tr in t.Rows)
        {
            foreach (TableCell tc in tr.Cells)
            {
                foreach (Control c in tc.Controls)
                {
                    if (c.GetType() == typeof(TextBox))
                    {
                        textbox = (TextBox)c;



                        Response.Write("TextBox Value is:" + textbox.Text + "<br/>");
                       
        
                    }
                }
            }
        }
    }

This code wotk in page that doest not inherit the master page but not in a contentPage, I am getting this error:

Quote:

Object reference not set to an instance of an object.

...
It is pointing to this line of code :

Quote:

Table t = (Table)Page.FindControl("pnlDate").FindControl("tb1");

Please can some tell me why and how to resolve this issue.