Noticed this while reviewing #13529, but it's also in master @ 5ba77df15. On macOS 10.13.6:

```
bitcoin-qt -disablewallet
```

<img width="1354" alt="schermafbeelding 2018-08-01 om 16 03 41" src="https://user-images.githubusercontent.com/10217/43526506-b23264d8-95a4-11e8-81a0-5debcad838ae.png">

The usual progress modal doesn't appear when QT starts, and clicking on the progress bar has no effect. It works fine when there's a wallet present.
{
    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)