System.NullReferenceException: Excel C#

I have written a class that deals with opening, writing to, and closing an Excel file.

The methods in the Excel class are being called by a different call. When I call the addDatatoExcel method the first time, everything is working, however when I call it again, I get a "System.NullReferenceException: Excel C#". VB says I am passing a null object. I know what the error is I just can not figure out how to prevent it.

public class ExcelFile
{
    public static ExcelFile C1;
    private string excelFilePath = "‪‪C:\\Final.xlsx";
    private int rowNumber = 1; // define first row number to enter data in Excel

    Excel.Application excelApp;
    Excel.Workbook excelWorkbook;
    Excel.Worksheet excelWorksheet;
    int ExcelCounter = 0;
    int checker = 0;
    string str4 = "h";
    
    public   void openExcel()
    {
        excelApp = null;

        excelApp = new Excel.Application(); // create Excel App
        excelWorkbook = excelApp.Workbooks.Add();
        
        excelWorksheet = (Excel.Worksheet)excelWorkbook.Sheets.Add();
    }

    public void addDataToExcel(string str4)
    {
        excelWorksheet.Cells[5, 1] = str4;
                      
        ExcelCounter++;
    }

    public void closeExcel()
    {
        excelWorkbook.Close();
        excelApp.Quit();

        System.Runtime.InteropServices.Marshal.FinalReleaseComObject(excelWorksheet);
        System.Runtime.InteropServices.Marshal.FinalReleaseComObject(excelWorkbook);
        System.Runtime.InteropServices.Marshal.FinalReleaseComObject(excelApp);

        GC.Collect();
        GC.WaitForPendingFinalizers();
    }
}

I want to be able to add data to the first same Excel sheet that was created.

This is the method that calls the functions in the Excel class.

public void AddTextToLabel(string str)
{
        ExcelFile _Excel = new ExcelFile();
        
        if (flag == 1) // Celsius 
        {
            //list Adding Function 1;                
            temp = Double.Parse(str);
            
            x++;
            list.Add(x, temp);
            zedGraphControl1.Invalidate();
            CreateChart(zedGraphControl1);
            //Example();
            
            if (Excelcounter == 0)
            {
                _Excel.openExcel();
                Excelcounter++;
            }
            
            _Excel.addDataToExcel(str);

            if (Excelcounter == 15)
            {
                _Excel.closeExcel();
            }
        }
}

I have checked and the variable str that is being passed to addDatatoExcel is not null. There are other parameters that change to null when the function is called the second time. Is this because I am not saving the Excel file between every call to AddDatatoExecl();

1st call to function

2nd call to function

I am new to C# so please be specific. Thanks