Object reference not set to an instance of an object.

Imports Microsoft.Office.Interop.Excel
Imports System.Text
Imports System.Reflection
Imports System.Data
Imports Excel = Microsoft.Office.Interop.Excel

Public Class Form1
    Private Const connstring As String = "Provider=Microsoft.Jet.OLEDB.4.0;" & _
        "Data Source=C:\Test.xls;Extended Properties=""Excel 8.0;HDR=YES;"""
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        'Dim objexcel As Microsoft.Office.Interop.Excel.Application
  
        ' Dim wb As Microsoft.Office.Interop.Excel.Workbook
        ' Dim ws As Microsoft.Office.Interop.Excel.Worksheet
        '  Dim mMsgResponse As Integer
        Dim objExcel As New Excel.Application
        Dim objBook As Excel.Workbook = objExcel.Workbooks.Open("C:\Users\Server\Downloads\DONEsb.xls")
        Dim objSheet As Excel.Worksheet
        Dim bolFlag As Boolean = True
        Dim excelRow As Integer = 7
        Dim excelCol As Integer = 1
        Dim strCell1 As String
        Dim strCell2 As String
        Dim strCell3 As String
        Dim strCell4 As String
        Dim strCell5 As String
        Dim strCell6 As String
        Dim dialog As New OpenFileDialog()

        ' If DialogResult.OK = dialog.ShowDialog Then
        'TextBox1.Text = dialog.FileName
        ' objExcel = New Microsoft.Office.Interop.Excel.Application
        ' objBook = objExcel.Workbooks.Open(TextBox1.Text)


        If Convert.ToString(objSheet.Cells(excelRow, 1).value) = "" Then
            bolFlag = False
            strCell1 = CType(objSheet.Cells(excelRow, 1).value, String)
            strCell2 = CType(objSheet.Cells(excelRow, 2).value, String)
            strCell3 = CType(objSheet.Cells(excelRow, 3).value, String)
            strCell4 = CType(objSheet.Cells(excelRow, 4).value, String)
            strCell5 = CType(objSheet.Cells(excelRow, 5).value, String)
            strCell6 = CType(objSheet.Cells(excelRow, 6).value, String)

            excelRow += 1
            strCell1 = ""
            strCell2 = ""
            strCell3 = ""
            strCell4 = ""
            strCell5 = ""
            strCell6 = ""


            objExcel.Visible = True

            objBook.Activate()


            MsgBox("")

        End If
        ' End If


        '   Catch ex As COMException

        '   MessageBox.Show("Error accessing Excel: " + ex.ToString())

    End Sub



End Class