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)
QDialog(parent),
    ui(new Ui::OptionsDialog),
    model(nullptr),
    mapper(nullptr)
{
    ui->setupUi(this);
    
    ui->databaseCache->setMinimum(nMinDbCache);
    ui->databaseCache->setMaximum(nMaxDbCache);
    ui->threadsScriptVerif->setMinimum(-GetNumCores());
    ui->threadsScriptVerif->setMaximum(MAX_SCRIPTCHECK_THREADS);
    ui->pruneWarning->setVisible(false);
    ui->pruneWarning->setStyleSheet("QLabel { color: red; }");
    ui->pruneSize->setEnabled(false);
    connect(ui->prune, &QPushButton::toggled, ui->pruneSize, &QWidget::setEnabled);
    
#ifndef USE_UPNP
    ui->mapPortUpnp->setEnabled(false);
#endif
    ui->proxyIp->setEnabled(false);
    ui->proxyPort->setEnabled(false);
    ui->proxyPort->setValidator(new QIntValidator(1, 65535, this));
    ui->proxyIpTor->setEnabled(false);
    ui->proxyPortTor->setEnabled(false);
    ui->proxyPortTor->setValidator(new QIntValidator(1, 65535, this));
    connect(ui->connectSocks, &QPushButton::toggled, ui->proxyIp, &QWidget::setEnabled);
    connect(ui->connectSocks, &QPushButton::toggled, ui->proxyPort, &QWidget::setEnabled);
    connect(ui->connectSocks, &QPushButton::toggled, this, &OptionsDialog::updateProxyValidationState);
    connect(ui->connectSocksTor, &QPushButton::toggled, ui->proxyIpTor, &QWidget::setEnabled);
    connect(ui->connectSocksTor, &QPushButton::toggled, ui->proxyPortTor, &QWidget::setEnabled);