Official OpcUa Viewer Example does not run out-of-the-box

void MainWindow::createClient()
{
    if (mOpcUaClient == nullptr) {
        mOpcUaClient = mOpcUaProvider->createClient(ui->opcUaPlugin->currentText());
        if (!mOpcUaClient) {
            const QString message(tr("Connecting to the given sever failed. See the log for details."));
            log(message, QString(), Qt::red);
            QMessageBox::critical(this, tr("Failed to connect to server"), message);
            return;
        }
        // ...
    }
}

    def createClient(self):
        if not self._opcua_client:
            self._opcua_client = self.mOpcUaProvider.createClient(self._ui.opcUaPlugin.currentText())
        if not self._opcua_client:
            qWarning("Connecting to the given server failed. See the log for details.")
            QMessageBox.critical(self,  "Failed to connect to server", message)
            return

        # ...