Aceessing controls in a Panel

I dynamically create some controls at runtime and add them into a panel.

Later on, I try to access those controls in a different function, butI get a null object error

Does anybody know what Im doing wrong?

Protected Sub TreeView1_SelectedNodeChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles TreeView1.SelectedNodeChanged
 
   Dim TextBox1 As New TextBox
   TextBox1.ID = "a"
   TextBox1.Style("Position") = "Absolute"
   Panel1.Controls.Add(TextBox1)
 
end sub
 
Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles btnSubmit.Click
 
        Dim txt As TextBox = New TextBox()
 
        txt = Panel1.FindControl("a")   'crash here
        Label1.Text = txt.Text
 
end sub

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