Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException without link to my own code

What could be the reason for this exception? I dont know what causes this. It appears at different situations. It seams pretty random. The program is still running without any limitations.

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.text.PlainView.paint(Unknown Source)
at javax.swing.text.FieldView.paint(Unknown Source)
at javax.swing.plaf.basic.BasicTextUI$RootView.paint(Unknown Source)
at javax.swing.plaf.basic.BasicTextUI.paintSafely(Unknown Source)
at javax.swing.plaf.basic.BasicTextUI.paint(Unknown Source)
at javax.swing.plaf.basic.BasicTextUI.update(Unknown Source)
at javax.swing.JComponent.paintComponent(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JComponent.paintChildren(Unknown Source)
at javax.swing.JComponent.paint(Unknown Source)
at javax.swing.JComponent.paintToOffscreen(Unknown Source)
at javax.swing.RepaintManager$PaintManager.paintDoubleBuffered(Unknown Source)
at javax.swing.RepaintManager$PaintManager.paint(Unknown Source)
at javax.swing.RepaintManager.paint(Unknown Source)
at javax.swing.JComponent._paintImmediately(Unknown Source)
at javax.swing.JComponent.paintImmediately(Unknown Source)
at javax.swing.RepaintManager$4.run(Unknown Source)
at javax.swing.RepaintManager$4.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.paintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.prePaintDirtyRegions(Unknown Source)
at javax.swing.RepaintManager.access$1200(Unknown Source)
at javax.swing.RepaintManager$ProcessingRunnable.run(Unknown Source)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEventImpl(Unknown Source)
at java.awt.EventQueue.access$500(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.awt.EventQueue$3.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)

Only this class uses AWT.

public class GraphicalUserInterface {
private int             currentLineNumber           = 0;
private Font            font;
private JFrame          frame                       = new JFrame("Database");
private Image           icon;
private JTextField      input                       = new JTextField();
private JTextPane       output                      = new JTextPane();
private JPanel          panel                       = new JPanel();
private int             pressedKey                  = 0;
private JScrollPane     scrollPane                  = new JScrollPane(panel);
private StyledDocument  styledDocument              = output.getStyledDocument();
private Object          synchronizerInputConfirm    = new Object();
private Object          synchronizerKeyInput        = new Object();
private Object          synchronizerKeyPressed      = new Object();
private JTextField      time                        = new JTextField();
private Timer           timer                       = new Timer();

public GraphicalUserInterface() throws FontFormatException, IOException, InterruptedException {
    Dimension dim = Toolkit.getDefaultToolkit().getScreenSize();
    ClassLoader classLoader = this.getClass().getClassLoader();
    InputStream inputStream = classLoader.getResourceAsStream("DejaVuSansMono.ttf");
    font = Font.createFont(Font.TRUETYPE_FONT, inputStream);
    font = font.deriveFont(Font.PLAIN, 15);
    inputStream.close();
    icon = new ImageIcon(classLoader.getResource("icon.png")).getImage();
    ActionListener inputListener = new ActionListener() {
        @Override public void actionPerformed(ActionEvent e) {
            synchronized (synchronizerInputConfirm) {
                synchronizerInputConfirm.notify();
            }
        }
    };
    KeyListener keyListener = new KeyListener() {
        @Override public void keyPressed(KeyEvent e) {
            pressedKey = e.getExtendedKeyCode();
            if (pressedKey == 8) {
                input.replaceSelection("");
            }
            else if (pressedKey == 10) {
                synchronized (synchronizerKeyInput) {
                    synchronizerKeyInput.notify();
                }
            }
            synchronized (synchronizerKeyPressed) {
                synchronizerKeyPressed.notify();
            }
        }

        @Override public void keyReleased(KeyEvent e) {}

        @Override public void keyTyped(KeyEvent e) {}
    };
    DocumentListener documentListener = new DocumentListener() {
        @Override public void changedUpdate(DocumentEvent e) {}

        @Override public void insertUpdate(DocumentEvent e) {
            synchronized (synchronizerKeyInput) {
                synchronizerKeyInput.notify();
            }
        }

        @Override public void removeUpdate(DocumentEvent e) {}
    };
    input.getDocument().addDocumentListener(documentListener);
    panel.setLayout(new BorderLayout(0, 0));
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setIconImage(icon);
    frame.setResizable(false);
    time.setEditable(false);
    time.setEnabled(false);
    time.setFont(font);
    time.setBorder(null);
    time.setBackground(Color.BLACK);
    time.setDisabledTextColor(Color.WHITE);
    panel.add(time, BorderLayout.NORTH);
    output.setEditable(false);
    output.setEnabled(false);
    output.setFont(font);
    output.setBorder(null);
    output.setBackground(Color.BLACK);
    output.setDisabledTextColor(Color.WHITE);
    panel.add(output, BorderLayout.CENTER);
    input.setBorder(null);
    input.setFont(font.deriveFont(Font.ITALIC));
    input.setCaretColor(Color.WHITE);
    input.setBackground(Color.BLACK);
    input.setForeground(Color.WHITE);
    input.addActionListener(inputListener);
    input.addKeyListener(keyListener);
    input.setSelectedTextColor(Color.BLACK);
    input.setSelectionColor(Color.WHITE);
    output.add(input, BorderLayout.AFTER_LAST_LINE);
    timer.scheduleAtFixedRate(new UpdateTime(time), 0, 500);
    scrollPane.setViewportBorder(null);
    scrollPane.setBorder(null);
    JScrollBar verticalScrollBar = scrollPane.getVerticalScrollBar();
    verticalScrollBar.setPreferredSize(new Dimension(0, 0));
    verticalScrollBar.setUnitIncrement(10);
    JScrollBar horizontalScrollBar = scrollPane.getHorizontalScrollBar();
    horizontalScrollBar.setPreferredSize(new Dimension(0, 0));
    horizontalScrollBar.setUnitIncrement(10);
    frame.setContentPane(scrollPane);
    panel.add(output);
    for (StringFormat format : StringFormat.values()) {
        format.initialise(styledDocument);
    }
    frame.setSize(800, 530);
    input.setSize(770, input.getFontMetrics(font).getHeight());
    frame.setLocation(dim.width / 2 - frame.getSize().width / 2, dim.height / 2 - frame.getSize().height / 2);
}