Login gmail open message check for sucess or fail in message

Java Selenium program Login to Gmail account in search box check for text this is working fine. How to check if the message is a success or fail If I write a separate class im getting session ID null exception.

//

    WebElement searchbox = driver.findElement(By.xpath("//input[@placeholder='Search mail']"));
    Thread.sleep(100);
    searchbox.click();
    searchbox.sendKeys("SAP to Magento stock sync");
    Thread.sleep(100);
    searchbox.sendKeys(Keys.ENTER);
    Thread.sleep(1000); 
    new WebDriverWait(driver, 30).until(ExpectedConditions
        .visibilityOfElementLocated(By.className("xY a4W")));
WebElement NumberofRecord = driver.findElement(By.className("xY a4W"));
System.out.println(NumberofRecord.getText());

error:

TimeoutException: Expected condition failed: waiting for visibility of element located by By.className: xY a4W (tried for 30 second(s) with 500 milliseconds interval)
    at org.openqa.selenium.support.ui.WebDriverWait.timeoutException(WebDriverWait.java:81)