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<!-- Describe the issue -->
double click bitcoin icon on the task bar with setting 'minimize to the tray' cause a blank window
<!--- What behavior did you expect? -->
![ezgif com-video-to-gif](https://user-images.githubusercontent.com/11154118/38133010-fad94d80-343f-11e8-8bf4-127100e9c1f4.gif)

<!--- What was the actual behavior (provide screenshots if the issue is GUI-related)? -->

<!--- How reliably can you reproduce the issue, what are the steps to do so? -->

<!-- What version of Bitcoin Core are you using, where did you get it (website, self-compiled, etc)? -->

<!-- What type of machine are you observing the error on (OS/CPU and disk type)? -->

<!-- Any extra information that might be useful in the debugging process. -->
<!--- This is normally the contents of a `debug.log` or `config.log` file. Raw text or a link to a pastebin type site are preferred. -->

return;
            }
            progressBarLabel->setText(tr("Connecting to peers..."));
            break;
    }
    QString tooltip;
    QDateTime currentDate = QDateTime::currentDateTime();
    qint64 secs = blockDate.secsTo(currentDate);
    tooltip = tr("Processed %n block(s) of transaction history.", "", count);
    
    if(secs < 90*60)
    {
        tooltip = tr("Up to date") + QString(".<br>") + tooltip;
        labelBlocksIcon->setPixmap(platformStyle->SingleColorIcon(":/icons/synced").pixmap(STATUSBAR_ICONSIZE, STATUSBAR_ICONSIZE));
#ifdef ENABLE_WALLET
        if(walletFrame)
        {
            walletFrame->showOutOfSyncWarning(false);
            modalOverlay->showHide(true, true);
        }
#endif 
        progressBarLabel->setVisible(false);
        progressBar->setVisible(false);
    }
    else
    {
        QString timeBehindText = GUIUtil::formatNiceTimeOffset(secs);
        progressBarLabel->setVisible(true);
        progressBar->setFormat(tr("%1 behind").arg(timeBehindText));
        progressBar->setMaximum(1000000000);