I opened the Log file, and it is mentionned that I pointed on nothing...
Nevertheless, I checked all my IDs in my DIAGRAM.XMI file and I found
nothing that could be problematic.
So I decided to check-out the previous version of my MODEL.XMI file, and
I'm re-drawing my diagram. I would spend less time that way than trying to
debug this error. And I really pay attention at every step of my diagram,
in order to work with something which as transparent as possible.