找不到错误：NullPointerException

Question

我正在获取NullPointerException并查找错误并阅读有关该错误的信息，但似乎仍无法解决问题。我的主要方法采用文件类型和字符串的列表，并将其传递给Parse方法以将Excel文件转换为txt文件。我是Java的新手，将不胜感激。谢谢！

Exception in thread "main" java.lang.NullPointerException
    at transform.Parse.Transform(Parse.java:54)
    at transform.Input.main(Input.java:62)

这是我的代码。我在显示异常的地方添加了两条注释：

package transform;

public class Input {

    private static final Object Face_value = null;

    public static void main( String args[] ) throws Exception {

        String str = " "; 
        Scanner sc = new Scanner(System.in);
        ArrayList<String> ExcelList = new ArrayList<String>();    
        System.out.println("Print .xlsx file names");
            while (sc.hasNextLine() && !(str = sc.nextLine()).equals("")) {
                ExcelList.add(str);
            } 

          for(int i=0; i<ExcelList.size(); i++) {
              System.out.println(ExcelList.get(i));
          }

        String str1 = " ";
        Scanner sc1 = new Scanner(System.in);
        ArrayList<String> txtList = new ArrayList<String>();
        System.out.println("Print desired .txt file names");  
        while(sc1.hasNextLine() && !(str1 = sc1.nextLine()).equals("")) {
            txtList.add(str1);
        }

        if(txtList.size() != ExcelList.size()) {
            System.out.println("The number of Excel files listed does not match the number of .txt files listed");
        }

        for(int i=0; i < txtList.size(); i++) {
            System.out.println(txtList.get(i));
        }

        //convert ArrayList of type String to type File
        for(int i=0; i < ExcelList.size(); i++) {
            File file = new File(ExcelList.get(i));
            Parse.Transform(file, txtList.get(i));
        }  
         ///////////////////////////////////////////////////////////////////   
         //ERROR ON THE LINE ABOVE(line 62) -- Parse.Transform(file, txtList.get(i));
         ///////////////////////////////////////////////////////////////////
         /////////////////////////////////////////////////////////////////// 
        System.out.println("\nDone");

}
 }




 package transform;

public class Parse{

public static void Transform (File ExcelFile, String TxtFile) {
Writer writer = null;

try {

        //read excel file and get the first sheet
        FileInputStream fis = new FileInputStream(ExcelFile);
        XSSFWorkbook wb = new XSSFWorkbook(fis);
        XSSFSheet sheet = wb.getSheetAt(0);

        // set the name of the .txt file
        File file = new File(TxtFile);        
        writer = new BufferedWriter(new FileWriter(file));
        Iterator rows = sheet.rowIterator();
        writer.write("@SecurityDeal\r\n");

      while( rows.hasNext() ) {
         XSSFRow row = (XSSFRow) rows.next();

         if(row.getRowNum() == 0){
           continue;
         }

          Iterator cells = row.cellIterator();

          XSSFCell Face_Value = row.getCell(10); 
          XSSFCell Rate = row.getCell(8);
          XSSFCell DealDate = row.getCell(6);
          XSSFCell MaturityDate = row.getCell(7);
          XSSFCell CReference = row.getCell(5);
          XSSFCell Comments = row.getCell(4);

          XSSFCell Counterparty = row.getCell(3);
         //////////////////////ERROR ON BELOW LINE(Line 54) ///////////////////////
         ////////////////////////////////////////////////////////////////
         ////////////////////////////////////////////////////////////////
         String Cp = Counterparty.toString(); 

          //convert date to correct format
          Date s1 = MaturityDate.getDateCellValue();
          SimpleDateFormat sdf = new SimpleDateFormat("M/dd/yyyy");
          String MD = sdf.format(s1);

          Date d1 = DealDate.getDateCellValue();
          SimpleDateFormat sdf1 = new SimpleDateFormat("M/dd/yyyy");
          String dd = sdf1.format(d1);


          //convert cells to String
          String Comments_String = Comments.toString();
          String Face_Value_String = Face_Value.toString();

          //convert strings to double
          double Number_Comments = Double.parseDouble(Comments_String);
          double Number_Face_Value = Double.parseDouble(Face_Value_String);

          //get rid of exponential format
          String Comments1 = String.format("%.0f", Number_Comments);
          String Face_Value1 = String.format("%.0f", Number_Face_Value);

          //Mapping for counterparty
          if(Cp.equals("CITI")){
             Cp = "US-Citibank, US";
          }
          else if(Cp.equals("BAML")){
             Cp = "US-Bank Of America Merrill Lynch";
          } 
          else if (Cp.equals("MIZUHO")) {
             Cp = "US-Mizuho Ltd";
          }
          else if(Cp.equals("WELLS")) {
             Cp = "US-Wells Fargo";                  
          }


          writer.write("$NEW\r\n" + "Instrument=US-Commercial Paper\r\n" + "BorrowInvest=BORROWING\r\n"
          + "Counterparty=" + Cp + "\r\n" + "Entity=US-GMF\r\n" + "Currency=USD\r\n" + "FaceValue=" + Face_Value1 + "\r\n"
          + "BaseRate=" + Rate + "\r\n" + "DealDate=" + dd + "\r\n" + "MaturityDate=" + MD + "\r\n" + "Dealer=GM_Financi\r\n" + 
          "CounterpartyReference=" + CReference + "\r\n" + "Comments=" + Comments1 + "\r\n$INSERT\r\n");  

          System.out.println("\r\n");
      }
      System.out.println("Successfully Created File");  
} catch ( IOException ex ) {
    ex.printStackTrace();
    System.out.println("Check formatting of file names. Make sure they end in .txt or .xlsx");
    } finally {
        try 
            {if (writer != null) {writer.close();}} 
                catch (IOException e) {e.printStackTrace();}
   }
  }
 }