You can do this in many ways before clicking on add items:
WebDriverWait wait = new WebDriverWait(driver, 40);
wait.until(ExpectedConditions.elementToBeClickable(By.id("urelementid")));// instead of id u can use cssSelector or xpath of ur element.
or
wait.until(ExpectedConditions.visibilityOfElementLocated("urelement"));
you can also wait like this. If you want to wait until invisible of previous page element:
wait.until(ExpectedConditions.invisibilityOfElementLocated("urelement"));
here is the link where u find all the selenium webdriver api that can be used for wait and its documentation.
https://selenium.googlecode.com/git/docs/api/java/org/openqa/selenium/support/ui/ExpectedConditions.html