A Nullreferenceexception was unhandled

Here is what I am trying to do.  I am trying to pull down the names of the tables located in an Excel spreadsheet from my VB.NET application.  After I load the Excel spreadsheet, I want my program to be able to located the table names, or worksheets in an Excel File.  I was able to see the table names in the schema viewer, but I am trying to figure out how to get those names into a string so that I can move those names into a drop down box.  I am getting a NullReferenceException was unhandled error at the line wher it says 'Excelsheets(i) = tblrow("Table_Name").ToString'.  Can somebody help me out please?  Below is my code.

    Private Sub btnGetSchema_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnGetSchema.Click
        Dim Path As String
        Dim conn_string As OleDbConnection
        Dim worksheets As New System.Data.DataTable
        Dim count_tables As Integer

        If TextBox1.Text = "" Then
            MsgBox("No spreadsheets have been loaded yet.")
            Exit Sub
        Else

            Path = TextBox1.Text
            conn_string = New OleDbConnection( _
                        "Provider= Microsoft.Jet.OLEDB.4.0;" & _
                        "Data Source =" & Path & ";" & _
                        "Extended Properties=""Excel 8.0;HDR=Yes;IMEX=1"";")
            Using (conn_string)
                conn_string.Open()
                worksheets = conn_string.GetSchema("Tables")

                count_tables = worksheets.Rows.Count

                MsgBox(count_tables)

                Dim Excelsheets() As String

                Dim i As Integer = 0
                Dim tblrow As DataRow
                While (i < count_tables)
                    Excelsheets(i) = tblrow("Table_Name").ToString
                    i = i + 1

                    MsgBox(Excelsheets(i))
                End While

            End Using

        End if


    End Sub