GET /model/{id} fails

Users that are not logged in get an Internal Server Error (500) when they try to access models.

To reproduce the error: What I did is that I logged in with my credentials and created a model, namely, http://enanomapper.ntua.gr:8080/model/d751fab1-1160-476c-b5e8-1a930a12df1e and I then logged out. I attempted to access the model I had previously created (of course this should fail because of insufficient access rights and return a status code 403 and an error report). However, I got a 500.