Prevent Physics from trying to add a body to Physics world when there is none present

src/tracks/track_object_manager.cpp

@@ -117,6 +117,7 @@ void TrackObjectManager::enable(std::string name)
 				curr->setEnable(true);
                 if (curr->getType() == "mesh")
                 {
+                    if (curr->getPhysicalObject() != NULL)
                     curr->getPhysicalObject()->addBody();
                 }