VS 2008 - Program To Recover Password If Lost

I've made a program. The program does what it's supposed to do, for a while. The program is meant to recover my password if I loose it, but when I test it, it fails. It works fine until it comes to the number of my password. Then this error shows: object reference not set to an instance of an object. And this section of code is highlighted:
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WebBrowser1.Document.GetElementById("ctl03$Login$username$input").SetAttribute("value", TextBox1.Text)

Here is complete Code:
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Public Class Form1
Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
Dialog1.ShowDialog()
[Code] .....