Erro ao carregar jcombobox do banco de dados

public class SubstanciaDao {
	List<Substancia>substancia;
	
	private Connection conexao;

    public SubstanciaDao() throws SQLException {
        this.conexao = CriaConexao.getConexao();

    }
public List<Substancia> getLista() throws SQLException{
        
        String sql = "select * from tb_substancias where nome like ? ORDER BY nome ASC";
        PreparedStatement stmt = this.conexao.prepareStatement(sql);
       
        ResultSet rs = stmt.executeQuery();
        
       
        List <Substancia> listaClientes = new ArrayList<Substancia>();
        
        while (rs.next()){
         
        	Substancia c1 = new Substancia();
                    c1.setFi_id(Integer.valueOf(rs.getString("fi_pk_id")));
                    c1.setSubstancia(rs.getString("fi_substancia"));
                    
                     
            listaClientes.add(c1);
        }    
        rs.close();
        stmt.close();
       
        return listaClientes;
         
        }
        

}

public class Combo extends JFrame {
	ArrayList <Substancia> listaSubstancia ;
	private JPanel contentPane;
	JComboBox cbSubstancia;

	/**
	 * Launch the application.
	 */
	public static void main(String[] args) {
		EventQueue.invokeLater(new Runnable() {
			public void run() {
				try {
					Combo frame = new Combo();
					
					frame.setVisible(true);
				} catch (Exception e) {
					e.printStackTrace();
				}
			}
		});
	}

	
	public Combo() throws SQLException {
		carregaCombo();                                                            // Carregar jcombobox ao inicializar
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		setBounds(100, 100, 450, 300);
		contentPane = new JPanel();
		contentPane.setBorder(new EmptyBorder(5, 5, 5, 5));
		contentPane.setLayout(new BorderLayout(0, 0));
		setContentPane(contentPane);
		
		JPanel panel = new JPanel();
		contentPane.add(panel, BorderLayout.CENTER);
		panel.setLayout(null);
		
		cbSubstancia = new JComboBox();
		cbSubstancia.setBounds(96, 43, 228, 23);
		panel.add(cbSubstancia);
	}
	
	public void carregaCombo()
	{
        DefaultComboBoxModel substancia = new DefaultComboBoxModel();
        for (int i=0;i< this.listaSubstancia.size();i++)
        {
        	substancia.addElement(this.listaSubstancia.get(i).getSubstancia());
        }
        cbSubstancia.setModel(substancia);
	}
}

java.lang.NullPointerException
	at Combo.carregaCombo(Combo.java:64)
	at Combo.<init>(Combo.java:44)
	at Combo$1.run(Combo.java:29)
	at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:641)
	at java.awt.EventQueue.access$000(EventQueue.java:84)
	at java.awt.EventQueue$1.run(EventQueue.java:602)
	at java.awt.EventQueue$1.run(EventQueue.java:600)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.AccessControlContext$1.doIntersectionPrivilege(AccessControlContext.java:87)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:611)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:269)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:184)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:174)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:169)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:161)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:122) :?:  :?: