message: Throw if source or destination are nullptr
diff: @@ -38,10 +38,7 @@ static bool validConnection(Structure* src, Structure* dst, Direction _d)
  {
  if (src == nullptr || dst == nullptr)
  {
  - #ifdef DEBUG
  - std::cout << "GraphWalker::validConnection() was passed a NULL Pointer." << std::endl;
  - #endif
  - return false;
  + throw std::runtime_error("GraphWalker::validConnection() was passed a NULL Pointer.");
  }
  if (_d == DIR_UP || _d == DIR_DOWN)
  {
  