Maliput Visualizer Crashes when viewing 2x2_intersection.yaml

[liangfok]

To replicate the problem, start Maliput visualizer:

./install/delphyne-gui/bin/maliput_viewer.sh

Then, from within the GUI, click on "Load" , change the file type to be *.yaml, then select 2x2_intersection.yaml. Once loaded, try to pan or zoom in. The visualizer will crash with the following error:

abort: Failure at /home/liang/dev/maliput_ws/src/delphyne_gui/delphyne-gui/visualizer/rules_visualizer_widget.cc:109 in OnRulesReceived(): condition 'items.size() == 1' failed.
./install/delphyne-gui/bin/maliput_viewer.sh: line 36:   422 Aborted                 (core dumped) visualizer --layout=${DELPHYNE_GUI_RESOURCE_ROOT}/layouts/layout_maliput_viewer.config "$@"