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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. -->

{
        frameBlocksLayout->addStretch();
        frameBlocksLayout->addWidget(unitDisplayControl);
        frameBlocksLayout->addStretch();
        frameBlocksLayout->addWidget(labelWalletEncryptionIcon);
        frameBlocksLayout->addWidget(labelWalletHDStatusIcon);
    }
    frameBlocksLayout->addStretch();
    frameBlocksLayout->addWidget(connectionsControl);
    frameBlocksLayout->addStretch();
    frameBlocksLayout->addWidget(labelBlocksIcon);
    frameBlocksLayout->addStretch();
    
    progressBarLabel = new QLabel();
    progressBarLabel->setVisible(false);
    progressBar = new GUIUtil::ProgressBar();
    progressBar->setAlignment(Qt::AlignCenter);
    progressBar->setVisible(false);
    
    
    
    QString curStyle = QApplication::style()->metaObject()->className();
    if(curStyle == "QWindowsStyle" || curStyle == "QWindowsXPStyle")
    {
        progressBar->setStyleSheet("QProgressBar { background-color: #e8e8e8; border: 1px solid grey; border-radius: 7px; padding: 1px; text-align: center; } QProgressBar::chunk { background: QLinearGradient(x1: 0, y1: 0, x2: 1, y2: 0, stop: 0 #FF8000, stop: 1 orange); border-radius: 7px; margin: 0px; }");
    }
    statusBar()->addWidget(progressBarLabel);
    statusBar()->addWidget(progressBar);
    statusBar()->addPermanentWidget(frameBlocks);