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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. -->

progressBar->setValue(nVerificationProgress * 1000000000.0 + 0.5);
        progressBar->setVisible(true);
        tooltip = tr("Catching up...") + QString("<br>") + tooltip;
        if(count != prevBlocks)
        {
            labelBlocksIcon->setPixmap(platformStyle->SingleColorIcon(QString(
                ":/movies/spinner-%1").arg(spinnerFrame, 3, 10, QChar('0')))
                .pixmap(STATUSBAR_ICONSIZE, STATUSBAR_ICONSIZE));
            spinnerFrame = (spinnerFrame + 1) % SPINNER_FRAMES;
        }
        prevBlocks = count;
#ifdef ENABLE_WALLET
        if(walletFrame)
        {
            walletFrame->showOutOfSyncWarning(true);
            modalOverlay->showHide();
        }
#endif 
        tooltip += QString("<br>");
        tooltip += tr("Last received block was generated %1 ago.").arg(timeBehindText);
        tooltip += QString("<br>");
        tooltip += tr("Transactions after this will not yet be visible.");
    }
    
    tooltip = QString("<nobr>") + tooltip + QString("</nobr>");
    labelBlocksIcon->setToolTip(tooltip);
    progressBarLabel->setToolTip(tooltip);
    progressBar->setToolTip(tooltip);
}
void BitcoinGUI::message(const QString &title, const QString &message, unsigned int style, bool *ret)