Karol,
the data-structure that underlies the 'navigation tree' you are using comes from an external source that was sufficient at the time, but has some deficiencies. To guard against future use-cases where it would not be sufficient I added warning triggers. At the time we were not certain that this would become part of the Mendix core technology stack.
You have managed to trigger the exact case this guard was put in place for several years ago.
The warning trigger was supposed to be a trigger for internal testing and a blocking flag for externals going to production. It is gratifying to see that this is what is happening now.
I suggest you file a ticket in MXDN so Mendix R&D can pick it up in the next release cycle.