Null pointer exception while trying to read xlsx file using selenium

I am trying to read excel file using selenium but null pointer exception is displayed. I got the duplicate question i tried that user answer as well still i am not able to resolve it. Below is my code.

package demopackage;

import java.io.File;

import java.io.FileInputStream;

import java.io.IOException;

import org.apache.poi.hssf.usermodel.HSSFWorkbook;

import org.apache.poi.ss.usermodel.Row;

import org.apache.poi.ss.usermodel.Sheet;

import org.apache.poi.ss.usermodel.Workbook;

import org.apache.poi.xssf.usermodel.XSSFWorkbook;

public class demo1 {

public void readExcel(String filePath, String fileName, String sheetName) throws IOException {

    // Create an object of File class to open xlsx file

    File file = new File(filePath + "\\" + fileName);

    // Create an object of FileInputStream class to read excel file

    FileInputStream inputStream = new FileInputStream(file);

    Workbook guru99Workbook = null;

    // Find the file extension by splitting file name in substring and getting only
    // extension name

    String fileExtensionName = fileName.substring(fileName.indexOf("."));

    // Check condition if the file is xlsx file

    if (fileExtensionName.equals(".xlsx")) {

        // If it is xlsx file then create object of XSSFWorkbook class

        guru99Workbook = new XSSFWorkbook(inputStream);

    }

    // Check condition if the file is xls file

    else if (fileExtensionName.equals(".xls")) {

        // If it is xls file then create object of HSSFWorkbook class

        guru99Workbook = new HSSFWorkbook(inputStream);

    }

    // Read sheet inside the workbook by its name

    Sheet guru99Sheet = guru99Workbook.getSheet(sheetName);

    // Find number of rows in excel file

    int rowCount = guru99Sheet.getLastRowNum() - guru99Sheet.getFirstRowNum();

    // Create a loop over all the rows of excel file to read it

    for (int i = 0; i < rowCount + 1; i++) {

        Row row = guru99Sheet.getRow(i);

        // Create a loop to print cell values in a row

        for (int j = 0; j < row.getLastCellNum(); j++) {

            // Print Excel data in console

            System.out.print(row.getCell(j).getStringCellValue() + "|| ");

        }

        System.out.println();
    }

}

// Main function is calling readExcel function to read data from excel file

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

    // Create an object of ReadGuru99ExcelFile class

    demo1 objExcelFile = new demo1();

    // Prepare the path of excel file

    String filePath = ("user.dir") + "\\src\\excelExportAndFileIO";

    // Call read file method of the class to read data

    objExcelFile.readExcel(filePath, "sheet1.xlsx", "ExcelGuru99Demo");

}
}

Output in console:

Exception in thread "main" java.lang.NullPointerException
    at demopackage.demo1.ReadExcel(demo1.java:38)
    at demopackage.demo1.main(demo1.java:59)

I tried everything but not able to resolve it also m trying it for the first time Thanks in advance