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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. -->

toggleHideAction(0),
    encryptWalletAction(0),
    backupWalletAction(0),
    changePassphraseAction(0),
    aboutQtAction(0),
    openRPCConsoleAction(0),
    openAction(0),
    showHelpMessageAction(0),
    trayIcon(0),
    trayIconMenu(0),
    notificator(0),
    rpcConsole(0),
    helpMessageDialog(0),
    modalOverlay(0),
    prevBlocks(0),
    spinnerFrame(0),
    platformStyle(_platformStyle)
{
    QSettings settings;
    if (!restoreGeometry(settings.value("MainWindowGeometry").toByteArray())) {
        
        move(QApplication::desktop()->availableGeometry().center() - frameGeometry().center());
    }
    QString windowTitle = tr(PACKAGE_NAME) + " - ";
#ifdef ENABLE_WALLET
    enableWallet = WalletModel::isWalletEnabled();
#endif 
    if(enableWallet)
    {
        windowTitle += tr("Wallet");