a gain! NullPointerException, need help

dear Master...

i meet " Exception in thread "main" java.lang.NullPointerException" in my code.
Before i post this tread, i have read some post about NullPointerException. I try to undersatanding it, but when i implement to my code, i still confuse... hahaha

finaly, i make a new post about myproblem. please for anybody to help me. What's wrong with mycode?

Code :

Exception in thread "main" java.lang.NullPointerException
        at aplikasiBarang.prosesCari(aplikasiBarang.java:575)
        at aplikasiBarang.tampilTabel(aplikasiBarang.java:608)
        at aplikasiBarang.<init>(aplikasiBarang.java:303)
        at aplikasiBarang.main(aplikasiBarang.java:780)

this is code for at aplikasiBarang.prosesCari(aplikasiBarang.java:575)

Code :

void prosesCari(String sql)
{
    tabelKosong();
    int a=1;
    try
    {
        Connection connect=new koneksiDb().condb();
        Statement stat = connect.createStatement();
        ResultSet rset = stat.executeQuery(sql);
        while(rset.next())
        {
            String kode=rset.getString(1);
....
...
..
.
 txtkode.setEnabled(false);
 tblbarang.setEnabled(false);
 tcari.setEnabled(false);
 bcari.setEnabled(false);
 bprev.setEnabled(false);
 bnext.setEnabled(false);
 teksFalse();
 tombolFalse();
 koneksiDb koneksi=new koneksiDb();
 koneksi.panggilDriver();
 tampilTabel(); // ==> when i click[COLOR="Blue"](aplikasiBarang.java:303)[/COLOR], the highlight foucus in here.
}

and the last is code for at aplikasiBarang.main(aplikasiBarang.java:780)

Code :

    public static void main(String[] args) {
        aplikasiBarang tampilan = new aplikasiBarang();
        tampilan.setTitle("Program Aplikasi Adventori Barang");
        tampilan.setSize(440,530);
        Dimension layar=Toolkit.getDefaultToolkit().getScreenSize();
        int l=(layar.width-tampilan.getSize().width)/2;
        int t=(layar.width-tampilan.getSize().width)/2;
        tampilan.setLocation(l,t);
        tampilan.setResizable(false);
        tampilan.setVisible(true);
        tampilan.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }