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)

{
    if (_optionsModel)
    {
        this->optionsModel = _optionsModel;
        
        connect(_optionsModel, &OptionsModel::displayUnitChanged, this, &UnitDisplayStatusBarControl::updateDisplayUnit);
        
        updateDisplayUnit(_optionsModel->getDisplayUnit());
    }
}
void UnitDisplayStatusBarControl::updateDisplayUnit(int newUnits)
{
    setText(BitcoinUnits::longName(newUnits));
}
void UnitDisplayStatusBarControl::onDisplayUnitsClicked(const QPoint& point)
{
    QPoint globalPos = mapToGlobal(point);
    menu->exec(globalPos);
}
void UnitDisplayStatusBarControl::onMenuSelection(QAction* action)
{
    if (action)
    {
        optionsModel->setDisplayUnit(action->data());
    }
}