When and why a directory called '_all1' (and index.html) is created for an non-existing instance of an entity?

[flyfoto]

The only non-empty property of that bogus object seems to be the .pk with a value '_all1'.
This has the effect that many accesses in the template to other properties of the object fail with hard to diagnose error messages:
e.g.
2024-07-05 13:52:10,690 statik.project INFO Generating Statik build...
2024-07-05 13:57:00,373 statik.project ERROR Template error: 'None' has no attribute 'state' (in file "e:\www\fly-foto2\fly-foto.de\website\templates\photo.html").
I had this effect twice but don't know how to avoid it.