On mac OS X 10.8.2:
Open the GUI,
Draw one place,
Go to the simulator,
Accept to upload the model to the server,
Go back to the editor,
A NullPointerException is now thrown and the GUI is from then unusable.
This is both a problem in the 32bit and the 64bit version (run on a 64bit system)
We also tested this on Linux Debian, where it does not seem to be a problem.