Visualizer: gui's LoadConfiguration option not working correctly

[francocipollone]

maliput/delphyne#806

Our ign-gui-based visualizer has an option to load configuration file (layout options) after the visualizer and the simulation has started.

However, when using that option there is some clear error when loading the Scene again:

• Interestingly if I close the window while the simulation is still on and I re-open the visualizer(without restarting the simulation) the visualization continues working well, the scene is correctly visualized. So this doesn't seem to be a problem of having the simulation already running.

• I used only the Scene3D plugin to narrow down the issue to the scene itself. Therefore, it isn't related to some plugin that depends on the Scene3d plugin and suddenly the latter is missing.