Forward port #534 to ros2 (#538)

Author: SteveMacenski

src/slam_toolbox_common.cpp

@@ -277,6 +277,7 @@ void SlamToolbox::publishVisualizations()
   while (rclcpp::ok()) {
     updateMap();
     if (!isPaused(VISUALIZING_GRAPH)) {
+      boost::mutex::scoped_lock lock(smapper_mutex_);
       closure_assistant_->publishGraph();
     }
     r.sleep();