Getting errors when running my Visual Basic application

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:

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************** 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)

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Here is the Actual Code, I'll Bold the actual lines the error is referring to. Thanks so much

Code:

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    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

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It looks like two lines are bolded. This one from the first snippet:

Array.Resize(ExcelList, Counter) <--- Line 147

and this one from the second:

For i = 0 To (ArrayString.Length - 1) <---Line 172

In the first case, the mistake was using an array in the first place. You should be using a List(of T), instead. Arrays don't change size. The List has .Add, Insert, and other methods for adding items, so the step you are taking of resizing the array downwards at the end isn't necessary if you use a list. If you want it to be an array afterwards, just use the .ToArray method of the List.

It also looks like you'd want a List(of Object), but if the values will always be something else, like a numeric type, then a list of that type would be better.

For the second bolded line, there's something wrong with the argument passed to the method. If it's a NullReferenceException, then what is wrong is that Nothing was passed to the sub, which would mean that ERX_Arrays.objArray is Nothing, unless the method is called from somewhere else.

Thank you so much for a quick response.

I think the reason an array was used is because in the other part of the code which is in the module not the form. When ever the program is opened, what it does is connects to a server on which there is a software installed that is used to communicate with difference devices so it has its own database and all. But what this code does is looks at specific path for the software to find all of the current devices added on there.
it goes down the list and takes the names from that program on that server and copies them into a combo box on this form. So whenever this program is opened up the combo box is populated with all of the device names.

The code that I have pasted here initially is the code when a button is pressed on the Form when the program is opened.

with this information, do you think using list would still be a better option?

Regarding the second case, the NullReferenceException. I figured as much from reading other articles that there is a null or a nothing that was passed from somewhere to somewhere. But I can't wrap my head around what was passed, from where, and to where.
Sorry for sounding dumb, i'm just so very new to all this and I dont write code all the time.

Thank you again so much for any help.

As a general rule, a list is the right choice if you don't know how many you will need, as it will efficiently expand as necessary. Having said that, the reason the List is efficient has to do with allocating memory. The solution used kind of skips that by allocating more than it needed, then trimming out what isn't necessary. That trimming step (the Resize) isn't great, but overall the approach does avoid the allocation issues.

However, you can do the same thing with a List, though it is rarely used. There is a constructor that takes an integer as a capacity. If you created a List(of whateverTypeMakesSense)(300), you'd have the same advantage of the current solution, but you wouldn't need the Resize, nor would it be useful.

As to the second part, don't feel too bad about that. It's going to take some digging for ANYONE to figure that one out. The first step would be to either put a breakpoint on the line that throws the exception, or just go look at the code when the debugger throws the exception. All you'd be doing is confirming that ArrayString is Nothing, though you barely need to confirm that, as it's the only possible cause for that exception on that line. The reason to confirm it is more just that it is good practice.

Once you have confirmed it, then the fun begins. I only see that function being called in this line:

Dim ERXByte = ASCIIsumOfString(ERX_Arrays.objArray)

You can look at the stack trace to be sure that's how you got there. Otherwise, you can right click on the function and Find All References (or something close to that). The point is that once you know how you got to the function, you have to figure out why the argument was Nothing. If you got there from that particular function call, then ERX_Arrays.objArray is Nothing. Why? Who knows, but I will say that the very name of the thing suggests that it's going to get ugly.