A page renders even when one of its data sets is empty

A page in MeshWeaver is assembled from several questions asked of your data at once: give me the products, give me the orders, give me the packages. Most pages ask for several kinds at a time, and it is completely normal for one of them to have nothing in it.

Asking for a kind of data the page does not stock is supposed to answer "nothing here" — and every page is written to expect exactly that answer, showing an empty list and carrying on. For one family of data types it answered with an error instead, and because that error surfaced while the page was being drawn, it did not just blank out one list: it replaced the whole page with "Rendering failed." Everything else on it — the parts with data, the parts that had nothing to do with the missing set — went away too.

The cause was two different address books that disagreed.

Before answering, the page's data workspace checks whether it knows the kind of data being asked for. That check was generous: if it did not recognise the exact kind, it accepted a close relative instead — a deliberate convenience elsewhere in the system, where a specialised record is filed under its general category. But the step that actually goes and fetches the data is strict, and looks only for the exact kind. So a request could pass the friendly check and then fail the strict one, and the mismatch became an error at the worst possible moment: mid-render.

Now both steps ask the same question. A kind of data the page genuinely stocks resolves exactly as before; one it does not gets the plain "nothing here" the page already knows how to display. A missing data set costs you an empty list, never the page.