Error - "Cannot invoke "org.openqa.selenium.WebElement.sendKeys(java.lang.CharSequence[])" because "this.UserName" is null

I am a newbie in Java and Selenium. Below are my java classes for POM, Driver Utilites and Test Case.

When I run the Test Case as TestNG test, I have : FAILED: verifyLogin java.lang.NullPointerException: Cannot invoke "org.openqa.selenium.WebElement.sendKeys(java.lang.CharSequence[])" because "this.userName" is null.

Please help me out. Let me know if further info is needed. Thanks in advance.

<----------------------------------------POM-------------------------------------------------->

package com.inetBankingV1.pageObjects;

import org.openqa.selenium.WebDriver;

import com.inetBankingV1.utilities.callBrowserDriver;

import org.openqa.selenium.WebElement;
import org.openqa.selenium.support.FindBy;
import org.openqa.selenium.support.How;
import org.openqa.selenium.support.ui.ExpectedConditions;
import org.openqa.selenium.support.ui.Wait;
import org.openqa.selenium.support.ui.WebDriverWait;

public class LoginPage {
    
    WebDriver driver;
    
    @FindBy(how=How.LINK_TEXT, using="Log in")
    WebElement logUser;
    
    @FindBy(how=How.NAME, using="username")
    WebElement userName;
    
    @FindBy(how=How.NAME, using="password")
    WebElement passWord;
    
    
    //constructor
    public LoginPage(WebDriver driver)
    {
        this.driver=driver;
    }
    
    public  void loginCheck(String uname,String passwd)
    {
        userName.sendKeys(uname);
        passWord.sendKeys(passwd);
        logUser.click();
    }

}

<----------------------------------------Driver------------------------------------------------>

    package com.inetBankingV1.utilities;

import java.util.concurrent.TimeUnit;

import org.openqa.selenium.WebDriver;
import org.openqa.selenium.chrome.ChromeDriver;
import org.openqa.selenium.firefox.FirefoxDriver;
import org.openqa.selenium.firefox.FirefoxOptions;
import org.openqa.selenium.remote.DesiredCapabilities;

public class callBrowserDriver {
    
    public WebDriver startApplication(String browser, String baseURL, WebDriver driver)
    
    {
    
    if(browser.equalsIgnoreCase("firefox"))
            {
        System.setProperty("webdriver.gecko.driver", "./Drivers/geckodriver.exe");
        FirefoxOptions firefoxOptions = new FirefoxOptions();
        firefoxOptions.setCapability("marionette", true);
         driver = new FirefoxDriver(firefoxOptions);
         
            }
    
    if(browser.equalsIgnoreCase("chrome"))
            {
        System.setProperty("webdriver.chrome.driver", "./Drivers/chromedriver.exe");
        driver=new ChromeDriver();
            }
    
    driver.manage().timeouts().pageLoadTimeout(30, TimeUnit.SECONDS);
    driver.manage().window().maximize();
    driver.navigate().to(baseURL);
    
    return driver;

}
    
}

<--------------------------------------Test Case----------------------------------------------->

    package com.inetBankingV1.testCases;

import com.inetBankingV1.utilities.callBrowserDriver;

import org.openqa.selenium.WebDriver;
import org.testng.annotations.Test;

import com.inetBankingV1.pageObjects.LoginPage;

public class TC2_VerifyLogin {

    WebDriver driver;
    callBrowserDriver browserDriver=new callBrowserDriver();

    @Test
    public void verifyLogin()
    {
        
        driver=browserDriver.startApplication("chrome","https://demosite.com/",this.driver);
        LoginPage login=new LoginPage(driver);

    login.loginCheck("admin","Password");   
    }
}

The issue is resolved after initialising the webelements of the POM class using initElements:

LoginPage login=PageFactory.initElements(driver, LoginPage.class);