problem; obtain value in detailsview cells

Question

Can  you tell me why I  can not access the value of a textbox in the detailsview control?

I am working on a new C# asp.net 2010 webforms  application. I am trying to obtain the value loaded into a textbox(cell) for a detailsview control. When I step through the code, the debugger says the value is NULL.  However when  I run new the web page, there is the correct value loaded into the cell(textbox).

When I am trying to access the cell, I am clicking on a button that is not inside of the detailsview control. This is the requestor's preference. If I need to use only the buttons in the detailsview control, please let me know and how would you code it?

The following is the code I have right now:

<%@ Page Language="C#" MasterPageFile="~/RpApp.Master" AutoEventWireup="true"
     CodeBehind="Detail.aspx.cs" Inherits="Support.Att_Detail" %>

    <asp:LinqDataSource ID="LinqDataSource1" runat="server" ContextTypeName="Support.AttDataContext"
         Select="new (Att_id, Prior_Cutoff_Date)"
         TableName="Attestations" Where="Att_id == @Attid">
         <WhereParameters>
             <asp:SessionParameter Name="Att_id" SessionField="AttID" Type="Int32" />
         </WhereParameters>
     </asp:LinqDataSource>
  
    <asp:DetailsView ID="DetailsView1" runat="server" Height="50px" Width="125px" EnableModelValidation="True"
         AutoGenerateRows="False" DataSourceID="LinqDataSource1" BorderStyle="Solid" HorizontalAlign="Center">
         <Fields>
             <asp:TemplateField HeaderText="Att ID">
                 <ItemTemplate>
                     <asp:TextBox ID="TextBoxAttid" runat="server" Text='<%# Eval("Att_id") %>'
                         BorderStyle="None"></asp:TextBox>
                 </ItemTemplate>
             </asp:TemplateField>
                      
            <asp:TemplateField HeaderText="Prior Cutoff Date" >
               
                <EditItemTemplate>
                     <asp:TextBox ID="TextBox1" runat="server" Text='<%# Bind("Prior_Cutoff_Date") %>'></asp:TextBox>
                 </EditItemTemplate>
                  <ItemTemplate>
                     <asp:Label ID="Label1" runat="server" Text='<%# Bind("Prior_Cutoff_Date") %>'></asp:Label>
                  
                </ItemTemplate>
               
            </asp:TemplateField>
         </Fields>
     </asp:DetailsView>

      <asp:Button 
                                       ID="btnNext" 
                                       Text="NEXT"
                                       runat="server"
                                       onClick="NextButton"
                                        CssClass="RptFormButton" /> 

--------

     The following is the code in the aspx.cs file

protected void NextButton(object sender, EventArgs e)
         {
               
             

                TextBox tb = (TextBox)DetailsView1.FindControl("TextBox1.Text");
                 string name = DetailsView1.Rows[0].Cells[1];

}