Compile master, and run with `src/qt/bitcoin-qt -regtest -nowallet`. As expected, no icon is displayed. Load and unload a wallet. The `HD` icon remains.

On startup:
![on_startup](https://user-images.githubusercontent.com/863730/72410834-711b4c00-37a4-11ea-9a10-bfcf3db7ce68.png)

After a wallet load and unload:
![after_load_unload](https://user-images.githubusercontent.com/863730/72410838-737da600-37a4-11ea-8854-dd13301c0135.png)

{
    onDisplayUnitsClicked(event->pos());
}
void UnitDisplayStatusBarControl::createContextMenu()
{
    menu = new QMenu(this);
    for (const BitcoinUnits::Unit u : BitcoinUnits::availableUnits())
    {
        QAction *menuAction = new QAction(QString(BitcoinUnits::longName(u)), this);
        menuAction->setData(QVariant(u));
        menu->addAction(menuAction);
    }
    connect(menu, &QMenu::triggered, this, &UnitDisplayStatusBarControl::onMenuSelection);
}
void UnitDisplayStatusBarControl::setOptionsModel(OptionsModel *_optionsModel)