title: Apachi POI - Cell setCellValue thows NullPointerException
body: <p>When I am trying to update existing excel file I am encountering following error:</p>
  
  <pre><code>Exception in thread "main" java.lang.NullPointerException
      at xltest.main(xltest.java:28)
  </code></pre>
  
  <p>My code:</p>
  
  <pre><code>FileInputStream file = new FileInputStream(new File("C:\\Users\\onu\\test.xlsx"));
  
  XSSFWorkbook workbook = new XSSFWorkbook(file);
  XSSFSheet sheet = workbook.getSheetAt(0);
  
  //Update the value of cell
  Cell cell = sheet.getRow(0).getCell(3); // cell D1
  cell.setCellValue("onu"); // line 28 which throws NPE
  
  file.close();
  
  FileOutputStream outFile =new FileOutputStream(new File("C:\\Users\\onu\\test.xlsx"));
  workbook.write(outFile);
  outFile.close();
  </code></pre>