I don't see a call to Entity::BAUBackground(), so Entity::renderer wont get initialized, if that's not the problem, try a debugger and check if everything is initialized.
oh damn I'll edit this post because obviously I made a lot of errors trying to make it more understandable. The actual name of the Entity class is BAUBackground actually, but for understandability reasons I changed it to Entity. Let me edit
the weird thing is, there is a defaults.sav file which holds all the default settings, which includes an entity which is an electric wire, which is supposed to be a solid body. It loads its sprite as it should, but it does not load its collisions... from the same file