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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. -->

{
    QActionGroup *tabGroup = new QActionGroup(this);
    overviewAction = new QAction(platformStyle->SingleColorIcon(":/icons/overview"), tr("&Overview"), this);
    overviewAction->setStatusTip(tr("Show general overview of wallet"));
    overviewAction->setToolTip(overviewAction->statusTip());
    overviewAction->setCheckable(true);
    overviewAction->setShortcut(QKeySequence(Qt::ALT + Qt::Key_1));
    tabGroup->addAction(overviewAction);
    sendCoinsAction = new QAction(platformStyle->SingleColorIcon(":/icons/send"), tr("&Send"), this);
    sendCoinsAction->setStatusTip(tr("Send coins to a Bitcoin address"));
    sendCoinsAction->setToolTip(sendCoinsAction->statusTip());
    sendCoinsAction->setCheckable(true);
    sendCoinsAction->setShortcut(QKeySequence(Qt::ALT + Qt::Key_2));
    tabGroup->addAction(sendCoinsAction);
    sendCoinsMenuAction = new QAction(platformStyle->TextColorIcon(":/icons/send"), sendCoinsAction->text(), this);
    sendCoinsMenuAction->setStatusTip(sendCoinsAction->statusTip());
    sendCoinsMenuAction->setToolTip(sendCoinsMenuAction->statusTip());
    receiveCoinsAction = new QAction(platformStyle->SingleColorIcon(":/icons/receiving_addresses"), tr("&Receive"), this);
    receiveCoinsAction->setStatusTip(tr("Request payments (generates QR codes and bitcoin: URIs)"));
    receiveCoinsAction->setToolTip(receiveCoinsAction->statusTip());
    receiveCoinsAction->setCheckable(true);
    receiveCoinsAction->setShortcut(QKeySequence(Qt::ALT + Qt::Key_3));
    tabGroup->addAction(receiveCoinsAction);
    receiveCoinsMenuAction = new QAction(platformStyle->TextColorIcon(":/icons/receiving_addresses"), receiveCoinsAction->text(), this);
    receiveCoinsMenuAction->setStatusTip(receiveCoinsAction->statusTip());