如何处理空值异常？

如何解决如何处理空值异常？

这是一个主类：

   package hrmInfoModule;
    
    public class HrmMain {
    
        public static void main(String[] args) throws Throwable {
            
            Admin_UserMgmt AUser = new Admin_UserMgmt();
            AUser.login();
            AUser.AdminUsrMgmt();
            AUser.AdminAddEmployee();
            AUser.AdminDeleteEmployee();
            AUser.AdminRecordVerification();
        }
    }

这是家长班

    package hrmInfoModule;
    
    import java.util.concurrent.TimeUnit;
    
    import org.openqa.selenium.By;
    import org.openqa.selenium.WebDriver;
    import org.openqa.selenium.WebElement;
    import org.openqa.selenium.firefox.FirefoxDriver;
    
    public class Hrmlogin {
        WebDriver driver;
        
    public void login()
    {
        System.setProperty("webdriver.gecko.driver","D:\\Software\\geckodriver.exe");
        WebDriver driver = new FirefoxDriver();
        driver.manage().timeouts().implicitlyWait(60,TimeUnit.SECONDS);
        String Url="https://opensource-demo.orangehrmlive.com/";
        String ID ="Admin";
        String Pswrd="admin123";
        driver.get(Url);
        WebElement login = driver.findElement(By.id("txtUsername"));
        login.sendKeys(ID);
        WebElement paswrd = driver.findElement(By.id("txtPassword"));
        paswrd.sendKeys(Pswrd);
        WebElement btn = driver.findElement(By.id("btnLogin"));
        btn.click();
    }
    }

这是一个儿童班

package hrmInfoModule;

import java.util.List;

import org.openqa.selenium.By;
import org.openqa.selenium.WebDriver;
import org.openqa.selenium.WebElement;
import org.openqa.selenium.interactions.Actions;
import org.openqa.selenium.support.ui.Select;

public class Admin_UserMgmt extends Hrmlogin{
    
    WebDriver driver;
    public void AdminUsrMgmt() throws Throwable 
    {
     
        WebElement Admin = driver.findElement(By.xpath(("//*[@id='menu_admin_viewAdminModule']")));
        WebElement UseMgmt = driver.findElement(By.xpath("//*[@id='menu_admin_viewAdminModule']/parent::li//following-sibling::ul//li[1]//a[@id='menu_admin_UserManagement']"));
        WebElement UseMgmt2 = driver.findElement(By.xpath("//*[@id='menu_admin_viewAdminModule']//following-sibling::ul/li/ul/li/a"));
        Actions act= new Actions(driver);
        Thread.sleep(2000);
        act.moveToElement(Admin).moveToElement(UseMgmt).moveToElement(UseMgmt2).click().build().perform();
    }
}

错误

Exception in thread "main" java.lang.NullPointerException
    at hrmInfoModule.Admin_UserMgmt.AdminUsrMgmt(Admin_UserMgmt.java:18)
    at hrmInfoModule.HrmMain.main(HrmMain.java:9)

现在的问题是，每当我通过对象引用变量“ AUser”执行主类时，它都不会调用父类的登录方法以及子类的AdminUsrMgmt方法，并显示错误。

让我知道如何处理线程“主”中的异常 java.lang.NullPointerException