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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 (nProgress == 0)
    {
        progressDialog = new QProgressDialog(title, "", 0, 100);
        progressDialog->setWindowModality(Qt::ApplicationModal);
        progressDialog->setMinimumDuration(0);
        progressDialog->setCancelButton(0);
        progressDialog->setAutoClose(false);
        progressDialog->setValue(0);
    }
    else if (nProgress == 100)
    {
        if (progressDialog)
        {
            progressDialog->close();
            progressDialog->deleteLater();
        }
    }
    else if (progressDialog)
        progressDialog->setValue(nProgress);
}
void BitcoinGUI::setTrayIconVisible(bool fHideTrayIcon)
{
    if (trayIcon)
    {
        trayIcon->setVisible(!fHideTrayIcon);
    }
}
void BitcoinGUI::showModalOverlay()