Set value to TextBox in UserControl inside placeholder - asp.net

Question

I have an textbox inside an user control. I created dinamically this user control and load in placeholder.
But when I tried to assign a value to the textbox, I raised next below error:
Object reference not set to an instance of an object
This is the user control:
<%# Control Language="C#" AutoEventWireup="true" CodeBehind="IVT_FormClient.ascx.cs" Inherits="Evi.Sc.Web.Evi.IVT.Sublayouts.IVT_FormClient" %>
<asp:Panel ID="pnlContainer" runat="server">
<asp:TextBox ID="txtClientNumber" runat="server"></asp:TextBox>
</asp:Panel>
The access modifier is (In the user control):
public string TxtFirstName
{
get { return txtFirstName.Text; }
set { txtFirstName.Text = value; }
}
In the web form I have the control reference:
<%# Reference Control="~/Evi/IVT/Sublayouts/IVT_FormClient.ascx" %>
In the code behind of the user control is:
public partial class frm_VerifyIdentity : System.Web.UI.Page
{
IVT_FormClient ivtFormClient = new IVT_FormClient();
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
IVT_FormClient ivtFormClient = (IVT_FormClient)LoadControl("~/Evi/IVT/Sublayouts/IVT_FormClient.ascx");
Client UserClient = new Client();
UserClient = Load_ClientVerification(Server.HtmlEncode(Request.QueryString["ID"]).Trim());
if (UserClient != null)
{
ivtFormClient.TxtFirstName = UserClient.FirstName;
plhFormClient.Controls.Add(ivtFormClient);
}
}
}
}
The error occur is this line of code:
ivtFormClient.TxtFirstName = UserClient.FirstName;

Answer 1

Do not create an instance of a UserControl via constructor but with LoadControl as you have already done in Page_Load. However, you are doing that only if(!IsPostBack). Hence the control is instantiated the next postback via constructor.
Also, you have to recreate dynamic controls on every postback. I would suggest to add the UserControl delaratively to the page. You can hide/show it accordingly. Otherwise you need to create/add it always, best in Page_Init instead of Page_Load.
So this is not best-practise(just add it to the page) but should work as desired:
IVT_FormClient ivtFormClient = null;
protected void Page_Init(object sender, EventArgs e)
{
ivtFormClient =(IVT_FormClient)LoadControl("~/Evi/IVT/Sublayouts/IVT_FormClient.ascx");
Client UserClient = new Client();
UserClient = Load_ClientVerification(Server.HtmlEncode(Request.QueryString["ID"]).Trim());
if (UserClient != null)
{
ivtFormClient.TxtFirstName = UserClient.FirstName;
plhFormClient.Controls.Add(ivtFormClient);
}
}