My code is throwing error as “Exception in thread ”main“ java.lang.NullPointerException” while using POM class

I created a POM class as below

WebDriver driver;

    By username= By.xpath("//input[@id='UEID' and @name='login']");
    By password= By.xpath("//input[@class='inputtext' and @name='password']");
    By Lbutton= By.xpath("//input[@class='styled-button' and @name='enter']");
    By skip = By.xpath("//*[@id=\"ext-gen3\"]");

    public Login(WebDriver driver)
    {
        this.driver=driver;
    }

    public void loginpage(String abc, String xyz)
    {
        driver.findElement(username).sendKeys(abc);
        driver.findElement(password).sendKeys(xyz);
        driver.findElement(Lbutton).click();
    }

and the below code i am calling from my main method

Login l1=new Login(driver);
l1.loginpage("username","password");

Help me in resolving this error