selenium webdriver中的空指针异常 - TestNG - PageFactory

我正在尝试使用selenium webdriver，testng和页面工厂自动化 . 但我面临空指针异常 .

public class BrowserOpenClose extends Common {
    public WebDriver driver;

    @BeforeTest
    @Parameters("browser")
    public void initBrowser(String browser) {
        if (browser.equalsIgnoreCase("FF")) {
            System.setProperty("webdriver.firefox.marionette", "./gecko/geckodriver.exe");
            driver = new FirefoxDriver();
            driver.manage().window().maximize();
            driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
            driver.get("https://www.flipkart.com/");
        } else if (browser.equalsIgnoreCase("CH")) {
            System.setProperty("webdriver.chrome.driver", "./chrome/chromedriver.exe");
            driver = new ChromeDriver();
            driver.manage().window().maximize();
            driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
            driver.get("https://www.flipkart.com/");
        }
    }

    @AfterTest
    public void closeBrowser() {
        waitUntil(10);
        driver.quit();
    }
}

    public class Login extends BrowserOpenClose {

    @FindBy(xpath = "//a[text()='Log In']")
    private WebElement loginLink;

    @FindBy(xpath = "//input[@class='_2zrpKA']")
    private WebElement username;

    @FindBy(xpath = "//input[@class='_2zrpKA _3v41xv']")
    private WebElement password;

    @FindBy(xpath = "//button[@class='_3zLR9i _1LctnI _36SmAs']")
    private WebElement loginButton;

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

    public void openLoginDialog() {
        loginLink.click();
    }

    public void enterUsername(String name) {
        username.sendKeys(name);
    }

    public void enterPassword(String pass) {
        password.sendKeys(pass);
    }

    public void clickLogin() {
        loginButton.click();
    }

}

    public class SwitchTShirtPage extends BrowserOpenClose {
    WebDriver driver;
    Login login;

    @BeforeMethod
    public void initElements() {
        login = new Login(driver);
    }

    @Test
    public void switchToTShirt() {
        login.openLoginDialog();
        login.enterUsername("abc");
        login.enterPassword("xyz");
        login.clickLogin();
    }

}

错误跟踪日志：org.openqa.selenium.support.pagefactory.DefaultElementLocator.findElement（DefaultElementLocator.java:69）中的java.lang.NullPointerException，位于org.openqa.selenium.support.pagefactory.internal.LocatingElementHandler.invoke（LocatingElementHandler . java：38）at com.sun.proxy . $ Proxy9.click（Unknown Source）at com.login.test.Login.openLoginDialog（Login.java:50）at com.homepage.test.SwitchTShirtPage.switchToTShirt（SwitchTShirtPage.java ：28）at sun.reflect.NativeMethodAccessorImpl.invoke0（Native Method）at sun.reflect.NativeMethodAccessorImpl.invoke（Unknown Source）at sun.reflect.DelegatingMethodAccessorImpl.invoke（Unknown Source）at java.lang.reflect.Method.invoke（ org.testng.internal.InvokeTestMethod（Invoker . ）的org.test.invoke.invokeMethod（Invoker.java:646）中的org.testng.internal.MethodInvocationHelper.invokeMethod（MethodInvocationHelper.java:100）中的未知来源 . java：811）org.testng.i上的org.testng.internal.Invoker.invokeTestMethods（Invoker.java:1129）在org.testng.TestRunner的org.testng.TestRunner.privateRun（TestRunner.java:746）的org.testng.internal.TestMethodWorker.run（TestMethodWorker.java:112）上的nternal.TestMethodWorker.invokeTestMethods（TestMethodWorker.java:129） .run（TestRunner.java:600）org.testng.SuiteRunner.runTest（SuiteRunner.java：366）org.testng.SuiteRunner.runSequentially（SuiteRunner.java：361）atg.testng.SuiteRunner.privateRun（SuiteRunner . java：319）org.testng.SuiteRunner.run（SuiteRunner.java:268）org.testng.SuiteRunnerWorker.runSuite（SuiteRunnerWorker.java:52）at org.testng.SuiteRunnerWorker.run（SuiteRunnerWorker.java:86）at org.testng.TestNG.runSuitesSequentially（TestNG.java:1264）org.testng.TestNG.runSuitesLocally（TestNG.java:1189）org.testng.TestNG.runSuites（TestNG.java:1104）atg.testng.TestNG .run（TestNG.java:1076）org.testng.remote.AbstractRemoteTestNG.run（AbstractRemoteTestNG.java:126）位于org.testng.remote的org.testng.remote.RemoteTestNG.initAndRun（RemoteTestNG.java:152） . RemoteTestNG.ma在（RemoteTestNG.java:57）