Error; NullReferenceException and Object reference not set to an instance of an object

I?m practicing examples in For Dummies ASP.NET 2.0.  Code is below:

<%@ Page Language="VB" %>
<script runat="server">
Protected Sub Button1_Click(ByVal sender As Object, _
ByVal e As System.EventArgs)
        GridSqlDataSource.SelectParameters("LastName").DefaultValue = "%" + TextBox1.Text + "%"
End Sub
    Protected Sub GridView1_SelectedIndexChanged(ByVal sender As Object, ByVal e As System.EventArgs)
        DetailsSqlDataSource.SelectParameters("EmployeeID").DefaultValue = GridView1.SelectedValue
        DetailsView1.DataBind()
    End Sub
</script>
<html>
<body style="font-family:Verdana,Arial,Sans-Serif;font-size:12px;">
<form id="form1" runat="serv ...