I get this error, can you guys help? someone else wrote this code before I started working here and now its not working so im trying to figure it out. Thanks so much

Here is the error I get.
Code:
************** Exception Text **************
System.NullReferenceException: Object reference not set to an instance of an object.
   at ERX_Ad_Hoc.Form1.ASCIIsumOfString(String[] ArrayString) in P:\AD_HOC ERX POLLING\ERX Ad Hoc\Form1.vb:line 172
   at ERX_Ad_Hoc.Form1.OpenExcelWkbk_Click(Object sender, EventArgs e) in P:\AD_HOC ERX POLLING\ERX Ad Hoc\Form1.vb:line 147
   at System.Windows.Forms.Control.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnClick(EventArgs e)
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent)
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks)
   at System.Windows.Forms.Control.WndProc(Message& m)
   at System.Windows.Forms.ButtonBase.WndProc(Message& m)
   at System.Windows.Forms.Button.WndProc(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m)
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m)
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
Here is the Actual Code, I'll Bold the actual lines the error is referring to. Thanks so much

Code:
    Private Sub OpenExcelWkbk_Click(sender As Object, e As EventArgs) Handles OpenExcelWkbk.Click
        Dim xlApp As Object = CreateObject("Excel.Application") 'Creates Excel Object
        Dim xlWorkbook As Object = xlApp.Workbooks.Open("""G:\ERX Trending.xlsm""") 'Opens Workbook
        Dim xlSheet As Object = xlWorkbook.Worksheets("Historical Data") 'Opens Worksheets
        xlSheet.Range("B1").Value = ComboBox1.SelectedItem 'Puts ERX in ComboBox into ERX Selection in Spreadsheet
        Dim DateEnd As Date = Date.Today 'Gets Todays Date
        Dim DateStart As Date = DateEnd.AddDays(-1) 'Gets Yesterdays Date
        Dim Time = Date.Now.ToShortTimeString 'Gets current time
        xlSheet.Range("D1").Value = DateStart 'Sets Time/Date Options for Last 24 Hours
        xlSheet.Range("D2").Value = DateEnd 'Sets Time/Date Options for Last 24 Hours
        xlSheet.Range("F1").Value = Time 'Sets Time/Date Options for Last 24 Hours
        xlSheet.Range("F2").Value = Time 'Sets Time/Date Options for Last 24 Hours
        xlSheet = xlWorkbook.Worksheets("Lists") 'Moves to hidden "Lists" Worksheet
        Dim Counter = 0 'Creates Counter
        Dim CellValue = xlSheet.Range("A1").VALUE 'Get Value from cell A1 in the hidden "Lists" Worksheet
        Dim ExcelList(500) As String
        While CellValue <> Nothing 'This will loop while there is a value in cell A1
            CellValue = xlSheet.Range("A" & (Counter + 1).ToString).VALUE 'cell A1 value
            ExcelList(Counter) = CellValue
            Counter = Counter + 1
        End While
        Array.Resize(ExcelList, Counter)  <--- Line 147
        Dim ExcelByte = ASCIIsumOfString(ExcelList)
        Dim ERXByte = ASCIIsumOfString(ERX_Arrays.objArray)
        If ExcelByte <> ERXByte Then
            Counter = 0
            While Counter <= ERX_Arrays.objArray.Length - 1 'While Loop Updates ERX Spreadsheet List
                xlSheet.Range("A" & (Counter + 1).ToString).Value = ERX_Arrays.objArray(Counter)
                Counter = Counter + 1
            End While
        End If
        xlWorkbook.Save() 'Saves Workbook
        xlWorkbook.Close() 'Closes Workbook
        xlApp.Quit() 'Quits Excel
        xlSheet = Nothing 'Empties Excel Variables
        xlWorkbook = Nothing 'Empties Excel Variables
        xlApp = Nothing 'Empties Excel Variables
        System.GC.Collect() 'Ends Excel Processes
        Process.Start("EXCEL.EXE", """G:\ERX Trending.xlsm""") 'Starts UI for Excel Wkbk

    End Sub
    Private Function ASCIIsumOfString(ArrayString() As String)
        Dim sum As Int32
        Dim tempString As String
        For i = 0 To (ArrayString.Length - 1) <---Line 172
            tempString = ArrayString(i)
            For n = 1 To tempString.Length
                sum = sum + Convert.ToByte(tempString(n - 1))
            Next
        Next
        Return sum
    End Function