By copying the object (Object3D newOne = original.cloneObject()
and it was a simple 2-triangle square.
After that, I am just adding it to the world and building it:
world.addObject(newOne);
newOne.build(false);
But you know what? It stopped... It was yelling that error on me for the whole yesterday and when I reinstalled the game today it is not doing that any more.
Hmm... I came here for your help, because this was a new error for me after I used the same method for introducing new objects into the world over and over again for the last year without any problems. I probably did something differently and just didn't realize the moment I fixed that. It is little bit funky. Since I am using xml files saved in the asset directory I have to sometimes fully uninstall the app from the device and install it again to apply the changes, so maybe the problem was somewhere there in the assets.
If it will return, I'll study it more. But thanks, if you will have any clue, what I could do wrong I would appreciate your hint.