message: Add handling for an edge case in graph traversal
  This changelist adds handling for an edge case in graph traversal, where an edge references an invalid upstream element.
diff: @@ -114,7 +114,7 @@ GraphIterator& GraphIterator::operator++()
  // Traverse to the first upstream edge of this element.
  _stack.push_back(StackFrame(_upstreamElem, 0));
  Edge nextEdge = _upstreamElem->getUpstreamEdge(_material, 0);
  - if (nextEdge)
  + if (nextEdge && nextEdge.getUpstreamElement())
  {
  extendPathUpstream(nextEdge.getUpstreamElement(), nextEdge.getConnectingElement());
  return *this;
  @@ -141,7 +141,7 @@ GraphIterator& GraphIterator::operator++()
  if (parentFrame.second + 1 < parentFrame.first->getUpstreamEdgeCount())
  {
  Edge nextEdge = parentFrame.first->getUpstreamEdge(_material, ++parentFrame.second);
  - if (nextEdge)
  + if (nextEdge && nextEdge.getUpstreamElement())
  {
  extendPathUpstream(nextEdge.getUpstreamElement(), nextEdge.getConnectingElement());
  return *this;
  