It seems that you need to wait for the page to be reloaded before clicking on the "Add" button.
In this case you could wait for the "Add Item" element to become stale before clicking on the reloaded element:
WebDriverWait wait = new WebDriverWait(driver, 20);
By addItem = By.xpath("//input[.='Add Item']");
// get the "Add Item" element
WebElement element = wait.until(ExpectedConditions.presenceOfElementLocated(addItem));
//trigger the reaload of the page
driver.findElement(By.id("...")).click();
// wait the element "Add Item" to become stale
wait.until(ExpectedConditions.stalenessOf(element));
// click on "Add Item" once the page is reloaded
wait.until(ExpectedConditions.presenceOfElementLocated(addItem)).click();