I want to catch a popup message and get its text content

If at least one field is empty, then when you click on the "Login" button, we receive a popup message "Please fill out this field". I need to check if this message appears. But I cannot capture it in the usual way.

    WebDriver driver = new ChromeDriver();
    driver.get("https://gdcloud.ru/release-17");
    driver.findElement(By.id("login_button")).click(); 
    Alert alert = driver.switchTo().alert();
    String alertMessage = alert.getText();
    System.out.println("message: " + alertMessage);

What am I doing wrong?

That is not the regular alert box that you could handle with driver.switchTo().alert();. It is just the HTML5 form validation which comes into action when you put "required" as the attribute of textfield and hence you cannot validate this with the help regular selenium code. But there is a workaround. Either you can use sikuli/autoit to check the presence of that tooltip message but remember, for different browsers, the tooltip look different. So either stick to one browser only or check for the driver that you are using and based on that check for that presence of image(tooltip) on the screen with sikuli/autoIt.