Java.lang.NullPointerException após separar código

private class Aba extends JPanel
    {
        private InterfacePartitura painelAbas;

        public Aba(String titulo, String autor, InterfacePartitura painel)
        {
            this.painelAbas = painel;

            this.setLayout(new FlowLayout(FlowLayout.LEFT, 0, 0));
            this.setBorder(BorderFactory.createEmptyBorder(2,0,0,-2));
            this.setOpaque(false);

            JLabel label = new JLabel(titulo + " - " + autor);
            label.setBorder(BorderFactory.createEmptyBorder(0,0,0,5));

            BotaoAba botao_fechar = new BotaoAba();

            this.add(label);
            this.add(botao_fechar);
        }

        private class BotaoAba extends JButton implements ActionListener
        {
            public BotaoAba()
            {
                this.setPreferredSize(new Dimension(17,17));
                this.setToolTipText("Fechar aba");
                this.setUI(new BasicButtonUI());
                this.setContentAreaFilled(false);
                this.setFocusable(false);
                this.setBorder(BorderFactory.createEtchedBorder());
                this.setBorderPainted(false);
                this.setRolloverEnabled(true);
                this.addMouseListener(buttonMouseListener);
                this.addActionListener(this);
            }

            public void actionPerformed(ActionEvent evento)
            {
                int i = painelAbas.indexOfTabComponent(Aba.this);
                fechaPartitura(i);
            }

            protected void paintComponent(Graphics g)
            {
                super.paintComponent(g);
                Graphics2D g2 = (Graphics2D) g.create();
                //shift the image for pressed buttons
                if (getModel().isPressed()) g2.translate(1, 1);
                g2.setStroke(new BasicStroke(2));
                g2.setColor(Color.BLACK);
                if (getModel().isRollover()) g2.setColor(Color.MAGENTA);
                int delta = 6;
                g2.drawLine(delta, delta, getWidth() - delta - 1, getHeight() - delta - 1);
                g2.drawLine(getWidth() - delta - 1, delta, delta, getHeight() - delta - 1);
                g2.dispose();
            }
        }

        private final MouseListener buttonMouseListener = new MouseAdapter()
        {
            public void mouseEntered(MouseEvent e)
            {
                Component component = e.getComponent();
                if (component instanceof AbstractButton)
                {
                    AbstractButton button = (AbstractButton) component;
                    button.setBorderPainted(true);
                }
            }

            public void mouseExited(MouseEvent e)
            {
                Component component = e.getComponent();
                if (component instanceof AbstractButton)
                {
                    AbstractButton button = (AbstractButton) component;
                    button.setBorderPainted(false);
                }
            }
        };
    }

    public int criaAba(String titulo, String autor, JTextArea area)
    {
        this.addTab(titulo + " - " + autor,area);
        
        Aba novaAba = new Aba(titulo,autor,this);

        this.setTabComponentAt(this.getSelectedIndex(),novaAba);
        this.setSelectedIndex(this.getTabCount() - 1);

        return this.getSelectedIndex();
    }

public void criaPartitura(String titulo, String autor, String clave, int tempo_compasso, int divisao_semibreve, String tom, int bpm, String comentarios)
    {
        JTextArea area = new JTextArea();
        
        int i = this.painel_partitura.criaAba(titulo,autor,area);

        Partitura part = new Partitura(titulo,autor,clave,tempo_compasso,divisao_semibreve,tom,bpm,comentarios);
        this.vetorPartituras.add(i,part);

        //Mostrar conteudo da partitura
        area.setText(titulo);
        area.append("\n" + autor);
        area.append("\n" + clave);
        area.append("\n" + tempo_compasso);
        area.append("\n" + divisao_semibreve);
        area.append("\n" + tom);
        area.append("\n" + bpm);
        area.append("\n" + comentarios);
    }