Check When Webpage Loaded And Fill In Form Within Loop

I am making this program that has a loop that goes to a website and fills in a form within the loop. But first, here is my code thus far

Dim message As String = TextBox3.Text
Dim loopnumber As Integer = TextBox4.Text 'makes the necessary variables
Dim browser As New WebBrowser
Dim url As String
[Code] .....

But what happens is that the loop runs a few times then it errors on the setattribute line (line 19 above) with
NullRefrenceException was unhandled
And I have a feeling that it is not loading the page fully before it tries to fill in the form.