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.
if (ev->type() == QEvent::ParentAboutToChange) {
        if (parent()) parent()->removeEventFilter(this);
    }
    else if (ev->type() == QEvent::ParentChange) {
        if (parent()) {
            parent()->installEventFilter(this);
            raise();
        }
    }
    return QWidget::event(ev);
}
void ModalOverlay::setKnownBestHeight(int count, const QDateTime& blockDate)
{
    if (count > bestHeaderHeight) {
        bestHeaderHeight = count;
        bestHeaderDate = blockDate;
    }
}
void ModalOverlay::tipUpdate(int count, const QDateTime& blockDate, double nVerificationProgress)