The part of the GUI that this setting is relevant for does not exist in a `--disable-wallet` build. 

![wallet_prefs](https://user-images.githubusercontent.com/863730/70331435-72547300-180d-11ea-842d-82f8529ae982.png)
QValidator(parent)
{
}
QValidator::State ProxyAddressValidator::validate(QString &input, int &pos) const
{
    Q_UNUSED(pos);
    
    CService serv(LookupNumeric(input.toStdString().c_str(), DEFAULT_GUI_PROXY_PORT));
    proxyType addrProxy = proxyType(serv, true);
    if (addrProxy.IsValid())
        return QValidator::Acceptable;
    return QValidator::Invalid;
}