How to handle modal dialog box in Selenium?

There is a scenario where I get the modal dialog box when I tried to login.

I am not able to handle login scenario because webdriver is not getting access to the modal dialog box. Is there any way to handle this?

See the details in below image:

public class TwitterLogin 
{

    static WebDriver driver;
    static String baseUrl="myURL";

    public static void main(String[] args)
    {
        driver= new FirefoxDriver();
        driver.get(baseUrl);
        driver.manage().window().maximize();

        //******Click on Login Link**********//
        WebElement LoginButton= driver.findElement(By.xpath("//button[@class='Button StreamsLogin js-login']"));
        LoginButton.click();

        WebElement UsernameTextbox=    driver.findElement(By.xpath("//input[contains(@name,'email]')]"));
        UsernameTextbox.clear();
        UsernameTextbox.sendKeys("uName");

        WebElement PasswdTextbox=driver.findElement(By.xpath("//input[contains(@type,'password')]"));
        PasswdTextbox.clear();
        PasswdTextbox.sendKeys("1234");

     }

}

After clicking on Login button, I'm not able to enter username & password.