java.langNullPointerException (query Mysql in Jframe Form)

I'm new to java programming MySQL I have made a database with Java and managed to insert data but I could not extract the information in certain fields.

This is the code to insert in Mysql:

 import java.awt.event.KeyEvent;
 import java.sql.*;
 import javax.swing.*;

 public class Usuario extends javax.swing.JFrame {

Connection conn = null;
PreparedStatement pst = null;
ResultSet rs = null;

private void btguActionPerformed(java.awt.event.ActionEvent evt) {                                     

    conn = MySqlConnect.ConnectDB();
    String Sql = "INSERT INTO usuarios (nombre, apellidos, nick, pw, usuario) values (?,?,?,?,?)";
    try {
        pst = conn.prepareStatement(Sql);
        pst.setString(1, txno.getText());
        pst.setString(2, txap.getText());
        pst.setString(3, txni.getText());
        pst.setString(4, txpa.getText());
        pst.setString(5, txus.getSelectedItem().toString());

        int rs = pst.executeUpdate();

        if (rs > 0) {
            JOptionPane.showMessageDialog(null, "Usuario registrado");
            limpiarus();

        } else {
            JOptionPane.showMessageDialog(null, "un error a ocurrido", "No se registro usuario", JOptionPane.ERROR_MESSAGE);
            limpiarus();
        }
    } catch (Exception e) {
        JOptionPane.showMessageDialog(null, e);
    }


}                                    

This would be the code to find the information of Mysql this has a combobox which I put "txus":

      private void btbuActionPerformed(java.awt.event.ActionEvent evt) {                                     

      conn = MySqlConnect.ConnectDB();

  String Sql = "SELECT *FROM usuarios where nick=?";
    try {

        pst = conn.prepareStatement(Sql);

        pst.setString(1, txni.getText());


        if (rs.next()){
        txno.setText(rs.getString("nombre"));
        txap.setText(rs.getString("apellidos"));
        txni.setText(rs.getString("nick"));
        txpa.setText(rs.getString("pw"));
        txus.setSelectedItem(rs.getString("usuario"));

        }else {
        JOptionPane.showMessageDialog(null, "incorrecto", "accso denegado", JOptionPane.ERROR_MESSAGE);

        }

    } catch (Exception e) {
        JOptionPane.showMessageDialog(null, e);
    }

This is my connection to mysql

 package Secure;
 import java.sql.*;
 import javax.swing.*;


public class MySqlConnect {

Connection conn=null;
public static Connection ConnectDB() {
try{

    Class.forName("com.mysql.jdbc.Driver");
    Connection conn=DriverManager.getConnection("jdbc:mysql://localhost:3306/basededatos", "root","1002478900k");
      JOptionPane.showMessageDialog(null, "Conectado a la base de datos");
      return conn;
}
catch (Exception e){
JOptionPane.showMessageDialog(null, e);
return null;
}

}
}

When executing the button it shows me an error java.langNullPointerException apparently the main problem is in the code of the search button.

Sorry for not summarizing the question but I feel it is necessary.

Thanks in advance and have a nice day.