Segfault due to null entities in the World Model

[LFBFerreira]

I recently moved to Ubuntu 14 and everything seems to be working, but i also seem to be having a lot more segfaults than i remember. After some debugging in QTcreator and GDB the segfaults seem to come on parts where a "const ed::EntityConstPtr& e" is being acessed but its null (not sure if can trust qtcreator's gdb, i've had wrong results before)

I've tried to put some checks to make sure the pointer in not NULL with this commit, but it didn't solve everything and it might cause some unexpected behaviour, still its better than just a segfault ( c38e42c )