Catch NPEs in model inferrer code

By switching the style and shape generator code to be part of the model 
inferrer, code generation / model inferring is triggered while editing (instead 
by saving), so when there is currently an invalid model this causes a lot of 
NPEs in the model inferrer code resulting in nasty error message pop ups in the 
Xtext editor. So this spots in the code have to handle all scenarios when a 
elements are not yet set.

Original issue reported on code.google.com by de.abg.r...@gmail.com on 20 Sep 2012 at 7:13

alexroldugin / spray

Catch NPEs in model inferrer code #170