Focus on page respective Textbox after user control button click

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C#

ASP.NET

C#4.0

Hi all

I am having a 3 textbox in my webform inside the master page . when text changed of 2nd or 3rd or 1st happens one usercontrol will open which contains one button.When i click the button in user control ,the respective text changed text box should be focused. I tried the following code

In UserControl
public event EventHandler ButtonClickDemo;

protected void imgPopupClose_Click(object sender, EventArgs e)
   {
       ButtonClickDemo(sender, e);
   }

In Web Page

In page
popMsg.ButtonClickDemo += new EventHandler(Demo1_ButtonClickDemo);
here popMsg is usercontrol

protected void Password_TextChanged(object sender, EventArgs e)
{
Session["s"] = "Password.ID;
popMsg.Message = "Please enter valid password";
popMsg.ShowPopUp();
}

protected void Demo1_ButtonClickDemo(object sender, EventArgs e)
    {
        string s = Session["s"].ToString();
        this.FindControl(s).Focus();//here am getting error as object reference not set to instance of the object

    }