C# Exception User-Unhandled

I am building a C# Excel Add-on to replace string xxx to yyy and find files in batch in a given folder path:

     string replace = "xxx";
     string replacement = "yyy";

     foreach (FileInfo file in listOfFiles)
            {
        foreach (Excel.Worksheet xlWorkSheet in xlWorkBook.Worksheets)
                {
                    Excel.Range r = (Excel.Range)xlWorkSheet.UsedRange;                    

                    Excel.Range first = r.Find(replace, m, m, Excel.XlLookAt.xlPart, Excel.XlSearchOrder.xlByRows, Excel.XlSearchDirection.xlNext, m, m, m);
                    if (first != null)
                    {
                        count++;
                        Excel.Range start = first;
                        do
                        {
                            start.Value = replacement;
                            count++;
                            start = r.FindNext(m);                            
                            xlWorkBook.Save();

                        }
                        while (start != first);
                    }
                }

                xlWorkBook.Close();
            }

But when I run the code, the start.Value = replacement pops an error of

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=<Cannot evaluate the exception source>
  StackTrace:
<Cannot evaluate the exception stack trace>

I don't see any problems with my code here. I already check if (first != null) before setting up start, so start will not be null.

Yes, you check for null, but you also have 2nd action which may lead to null.
This line will return null when there are no other matches

 start = r.FindNext(m);         

You should do your check after each re-asign of the start variable.