Blank music area

I just opened a file in Dorico 4 that I was working on successfully yesterday in Dorico 4 (created in probably Dorico 2-something a few years ago), and got this:

Yet the music information is all there, as Play Mode indicates:

And everything plays back fine. Here’s the project file:
Barcrofte O almighty God.dorico (682.6 KB)
This is definitely the strangest thing Dorico has ever done to me. Any ideas what’s going on here?

In Engrave mode, the Default Full Score page template set appeared not to contain any page templates – switching it to the Default Part set and back brought them back.

Cool, many thanks. I wonder how that happened.