Why does an empty excel cell give null pointer exception when compared with null using "=="?

Question

I'm trying to check whether cell of an empty excel worksheet is null or not by using "==" in if but getting Null pointer exception. Why? Is there a way to do it successfully. I've used POI JARs.

FileInputStream fi = new FileInputStream(new File(sss));

XSSFWorkbook workbook = new XSSFWorkbook(fi);

//Get first/desired sheet from the workbook
XSSFSheet sheet = workbook.getSheetAt(0);

row=sheet.getRow(0);
if(row.getCell(0)==null)
  System.out.println("Cell is null");
else
  System.out.println("Cell is not null");