Quantity Discounts Available pop-up window gives log error

Found this bug and just recreated on fresh install with demo products of Zen Cart v1.5.6c running PHP Version 7.2.20 (my host doesn't have 7.3 yet).
Every time you click the "Quantity Discounts Available" link and the "Option Quantity Discounts" window opens I get:

[07-Aug-2019 16:54:54 UTC] Request URI: /zen-cart-v1.5.6c/index.php?main_page=popup_attributes_qty_prices&products_id=159&products_tax_class_id=1, IP address: ...*
#1 require(/home/server/public_html/zen-cart-v1.5.6c/includes/modules/pages/popup_attributes_qty_prices/header_php.php) called at [/home/server/public_html/zen-cart-v1.5.6c/index.php:36]
--> PHP Warning: Use of undefined constant NAVBAR_TITLE_1 - assumed 'NAVBAR_TITLE_1' (this will throw an Error in a future version of PHP) in /home/server/public_html/zen-cart-v1.5.6c/includes/modules/pages/popup_attributes_qty_prices/header_php.php on line 12.

Appears that (unless there is a need to change the expected functionality of that line) there is a file missing for this main_page=xxxx condition: includes/languages/english/popup_attributes_qty_prices.php that would contain at least the definition missing...