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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. -->

{
    
    m_handler_message_box->disconnect();
    m_handler_question->disconnect();
}
void BitcoinGUI::toggleNetworkActive()
{
    m_node.setNetworkActive(!m_node.getNetworkActive());
}
UnitDisplayStatusBarControl::UnitDisplayStatusBarControl(const PlatformStyle *platformStyle) :
    optionsModel(0),
    menu(0)
{
    createContextMenu();
    setToolTip(tr("Unit to show amounts in. Click to select another unit."));
    QList<BitcoinUnits::Unit> units = BitcoinUnits::availableUnits();
    int max_width = 0;
    const QFontMetrics fm(font());
    for (const BitcoinUnits::Unit unit : units)
    {
        max_width = qMax(max_width, fm.width(BitcoinUnits::longName(unit)));
    }
    setMinimumSize(max_width, 0);
    setAlignment(Qt::AlignRight | Qt::AlignVCenter);
    setStyleSheet(QString("QLabel { color : %1 }").arg(platformStyle->SingleColor().name()));
}
void UnitDisplayStatusBarControl::mousePressEvent(QMouseEvent *event)