Null Reference Exception on Textbox

Question

I have code that when an item from the first dropdownlist is selected, two dropdownlists and a textbox is populated. I added a blank item at the beginning of the first dropdown. Without the blank item, the code works fine. However, with the blank item, the ddls are populated, except the textbox, which gives me a NullReferenceException error.

The error points at:

TextBox1.Text = Convert.ToString(DropDownList2.SelectedItem.Value)

I can't pinpoint the problem here.

Here is my code:

 Imports System.Data
 Imports System.Data.SqlClient
 Imports System.Configuration
 Imports System.Web.UI.WebControls

Partial Class companydropdown
Inherits System.Web.UI.Page

Private Sub Page_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
    'Put user code to initialize the page here
    DropDownList1.Items.Add("")
End Sub

Private Sub BindDropDownList(DropdownList1 As DropDownList, query As String, text As String, value As String, defaultText As String)


    If Not IsPostBack Then
        ' Read sql server connection string from web.config file

        Dim sConstr As String = ConfigurationManager.ConnectionStrings("ds17701ConnectionString").ConnectionString
        Dim Conn As New SqlConnection(sConstr)
        Dim cmd As SqlCommand = New SqlCommand("sspCompanyReportList", Conn)
        Dim dt As New DataSet()

        'Set up Connection object and Connection String for a SQL Client


        Using Conn
            Conn.Open()
            Dim comm As New SqlCommand("SELECT CompanyName FROM CompanyList ORDER BY CompanyName", Conn)
            cmd.CommandType = CommandType.StoredProcedure 'Setup Command Type
            cmd.CommandText = "sspCompanyReportList" ' Stored Procedure to Call
            cmd.CommandType = CommandType.StoredProcedure

            Dim da As New SqlDataAdapter(comm)
            da.Fill(dt)
        End Using

        DropdownList1.DataSource = dt
        DropdownList1.DataTextField = "CompanyName"
        DropdownList1.DataValueField = "CompanyID"
        DropdownList1.DataBind()

    End If
End Sub

Protected Sub DropDownList1_SelectedIndexChanged(sender As Object, e As EventArgs)

    DropDownList2.Enabled = False
    DropDownList3.Enabled = False

    If DropDownList1.SelectedIndex > 0 Then
        Dim query As String = String.Format("CompanyName", DropDownList1.SelectedIndex)
        DropDownList3.Enabled = True
        DropDownList2.Enabled = True
        DropDownList1.Enabled = True
        TextBox1.Enabled = True

    Else

        DropDownList1.Items.Clear()

    End If
End Sub


Protected Sub DropdownList2_SelectedIndexChanged(sender As Object, e As System.EventArgs) Handles DropDownList1.SelectedIndexChanged

    If DropDownList1.SelectedIndex > 0 Then
        Dim sConstr As String = ConfigurationManager.ConnectionStrings("ds17701ConnectionString").ConnectionString
        Dim Conn As New SqlConnection(sConstr)
        Dim dt As New DataSet()
        Dim valsql As String = ""


        valsql = "SELECT [CompanyID], [CompanyName], [MemberTypeID], [MembershipStatus], [GroupID]  FROM CompanyList WHERE COMPANYID = " & DropDownList1.SelectedValue

        Using Conn
            Conn.Open()
            Dim comm As New SqlCommand(valsql, Conn)
            Dim da As New SqlDataAdapter(comm)
            da.Fill(dt)
        End Using


        'DropDownList1.Items.Add(New ListItem(""))


        DropDownList2.DataTextField = "MemberTypeID"
        DropDownList2.DataValueField = "MembershipStatus"
        DropDownList3.DataTextField = "GroupID"
        DropDownList3.DataValueField = "GroupID"
        TextBox1.Text = Convert.ToString(DropDownList2.SelectedItem.Value)

        'Bind sql server data into the Dropdown List
        DropDownList2.DataBind()
        DropDownList3.DataBind()
        TextBox1.DataBind()
    Else

        DropDownList2.Items.Clear()
        DropDownList3.Items.Clear()
        TextBox1.Text.ToString()
    End If



End Sub


'Protected Sub Page_Load(sender As Object, e As EventArgs) Handles Me.Load

'End Sub
End Class