add runtime error check for null nexus feature

include/core/Layer.hpp

@@ -116,6 +116,9 @@ namespace ngen
                     //TODO in a DENDRITIC network, only one destination nexus per catchment
                     //If there is more than one, some form of catchment partitioning will be required.
                     //for now, only contribute to the first one in the list
+                    if(nexus == nullptr){
+                        throw std::runtime_error("Invalid (null) nexus instantiation downstream of "+id+". "+SOURCE_LOC);
+                    }
                     nexus->add_upstream_flow(response_m_h, id, output_time_index);
                     /*std::cerr << "Add water to nexus ID = " << nexus->get_id() << " from catchment ID = " << id << " value = "
                               << response << ", ID = " << id << ", time-index = " << output_time_index << std::endl; */