MaterialX Null Pointer Dereference in getShaderNodes due to Unchecked nodeGraph->getOutput return

// SNIP...
        else if (input->hasNodeGraphString())
        {
            // Check upstream nodegraph connected to the input.
            // If no explicit output name given then scan all outputs on the nodegraph.
            ElementPtr parent = materialNode->getParent();
            NodeGraphPtr nodeGraph = parent->getChildOfType<NodeGraph>(input->getNodeGraphString());
            if (!nodeGraph)
            {
                continue;
            }
            vector<OutputPtr> outputs;
            if (input->hasOutputString())
            {
                outputs.push_back(nodeGraph->getOutput(input->getOutputString())); // <--- null ptr is returned
            }
            else
            {
                outputs = nodeGraph->getOutputs();
            }
            for (OutputPtr output : outputs)
            {
                NodePtr upstreamNode = output->getConnectedNode(); // <--- CRASHES HERE
                if (upstreamNode && !shaderNodeSet.count(upstreamNode))
                {
                    if (!target.empty() && !upstreamNode->getNodeDef(target))
                    {
                        continue;
                    }
                    shaderNodeVec.push_back(upstreamNode);
                    shaderNodeSet.insert(upstreamNode);
                }
            }
        }
    }
// SNIP...