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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. -->

}
    
    if (!msgType.isEmpty())
        strTitle += " - " + msgType;
    
    if (style & CClientUIInterface::ICON_ERROR) {
        nMBoxIcon = QMessageBox::Critical;
        nNotifyIcon = Notificator::Critical;
    }
    else if (style & CClientUIInterface::ICON_WARNING) {
        nMBoxIcon = QMessageBox::Warning;
        nNotifyIcon = Notificator::Warning;
    }
    
    if (style & CClientUIInterface::MODAL) {
        
        QMessageBox::StandardButton buttons;
        if (!(buttons = (QMessageBox::StandardButton)(style & CClientUIInterface::BTN_MASK)))
            buttons = QMessageBox::Ok;
        showNormalIfMinimized();
        QMessageBox mBox(static_cast<QMessageBox::Icon>(nMBoxIcon), strTitle, message, buttons, this);
        mBox.setTextFormat(Qt::PlainText);
        int r = mBox.exec();
        if (ret != nullptr)
            *ret = r == QMessageBox::Ok;
    }
    else
        notificator->notify(static_cast<Notificator::Class>(nNotifyIcon), strTitle, message);
}
void BitcoinGUI::changeEvent(QEvent *e)