How do I save a QByteArray in my Qt panel?

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I am creating a RViz plug-in / Qt panel like in this tutorial.

There is a QTableWidget in this panel and I would like to be able to save/restore the geometry when the user closes/launches again the panel/application.

My save callback looks like this:

void DisplayTopics::save(rviz::Config config) const
                         {
  rviz::Panel::save(config);

  QByteArray table_header_state(table_->horizontalHeader()->saveState());
  // FIXME Saves an empty string, bug in RViz?
  config.mapSetValue("table_header_state", table_header_state);
}

The load callback looks like:

void DisplayTopics::load(const rviz::Config& config)
{
  rviz::Panel::load(config);

  QVariant table_header_state_var;
  if (config.mapGetValue("table_header_state", &table_header_state_var))
  {
    if (table_header_state_var.canConvert(QMetaType::QByteArray))
    {
      QByteArray table_header_state(table_header_state_var.toByteArray());

      if (!table_header_state.isEmpty())
        table_->horizontalHeader()->restoreState(table_header_state);
    }
  }
}

When the config file is saved, the table_header_state field is empty:

  - Class: topics_rviz_plugin/DisplayTopics
    Name: DisplayTopics
    table_header_state: ""

I have tested with qDebug and the QByteArray is not empty when saving.

Why is the field empty? Full code example is available here: https://gitlab.com/InstitutMaupertuis...