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>

<aspetailsView 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 CMS 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>
</aspetailsView>

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

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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];