Clicking on "+" in the States editor gets me SOFT ASSERTS an a cryptic error popup

I use a freshly set up QMLproject created with the Qt Creator wizard.

I open the States editor and click on the "+" there.

I get pages full of output in the console (incl. some soft asserts) and a cryptic error message with this contents:

'Invalid property name "states". (M16)'

No new state is added.