Web Forms :: How To Reference A Control On A Master Page From A Content Page Code-behind

I am attempting to access a textbox control on a master page from a code behind base class but having problems.

I have 3 levels of master pages. m1.master is the master page for m2.master which is the master page for m3.master.

m3.master has a textbox as shown below

<%Master
Language="C#"
MasterPageFile="~/m2.master"
AutoEventWireup="true"
CodeFile="M3.master.cs"
Inherits="M3"
%>
<asp:Content
ID="Content5"
ContentPlaceHolderID="M2"
Runat="Server">
<asp:TextBox
ID="text1"
runat="server"
></asp:TextBox>
<asp:ContentPlaceHolder
ID="M3"
runat="Server"></asp:ContentPlaceHolder>
</asp:Content>

I have a content page c1.aspx that uses m3.master as its master page as shown below:

<%@
Page
Title=""
Language="C#"
MasterPageFile="~/m3.master"
AutoEventWireup="true"
CodeFile="c1.aspx.cs"
Inherits="_c1"
%>
<%@
MasterType
VirtualPath="~/m3.master"
%>
<asp:Content
ID="Content1"
ContentPlaceHolderID="M3"
Runat="Server">
</asp:Content>

My content page codebehind class inherits from a base class BaseForm

public partial
class
_c :
BaseForm{}

The problem that i am having is that I cannot access the texbox from the class BaseForm. The BaseForm code is as follows:

using System;
using System.Collections.Generic;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
public...........

I'm getting a null reference exception for txt1