[QGIS-Developer] Qt-Creator build: ModuleNotFoundError: No module named 'processing'

[Richard Duivenvoorde]

When I build QGIS with Qt-Creator to debug something, QGIS always starts
with the error:
ModuleNotFoundError: No module named 'processing'

Which is fine most of the time, as the rest runs fine, and because I did
not have to debug some processing thingies recently.

But now I want to debug a processing thing... so... anybody a pointer
why processing isn't picked up?
Should I put a custom python path?
I already did a fresh configure and cleaned up everything.

TIA,
Richard Duivenvoorde