PurchaseOrder has an empty PopupContents.tpl which causes failure to load
Create a UIType 10 field for a Purchase Order.
Then go add a new record (either as a main screen or as a quick create) and click in the icon to load the popup for the Purchase Order contents so you can select the correct record to link.
You get an empty Popup that displays the text TEMPLATE: layout/modules/PurchaseOrder/PopupContents.tpl
.
This file https://code.vtiger.com/vtiger/vtigercrm/blob/7.4.0/layouts/v7/modules/PurchaseOrder/PopupContents.tpl should not exist. If you remove it or rename it, then the problem goes away and the popup loads correctly.