method always visible by other class

Then I have a class called MyDBConn.java, inside this class there's a
method called GetResultFromPazienti():

MyDBConn.java:
==================================

package cc;

import java.sql.*;

public class MyDBConn {
    private Connection myConnection;
    private java.sql.Statement stmt;

// ...

public ResultSet getResultFromPazienti2(String query) {
        ResultSet rs=null;
        try{
            rs=stmt.executeQuery(query);
        }
        catch(Exception e){
            alerts.showErr(e.getMessage());
        }
        return rs;
    }

// ...

==================================

After this, I have a class called Paziente.java, from this class I
would call method: getResultFromPazienti2(), class is something like
this:

File: Paziente.java
==================================
package cc;
import java.sql.*;

public class Paziente extends javax.swing.JFrame {
        private MyDBConn mdbc;
        private java.sql.Statement stmt;

// ...
// ...

    private void formWindowOpened(java.awt.event.WindowEvent evt)
{
        ResultSet rs=mdbc.getResultFromPazienti2("select ... from ...
where ...");

        try {
            rs.next();
                 // ...
                 txtNome.setText(rs.getString("NOME"));
                 // ...
        } catch (SQLException ex) {
            ex.printStackTrace();
        }
    }
}

==================================

At first moment seem that are not problems, infact there's no problem
in compile time, but when I execute project and method
formWindowOpened() is started an "Exception in thread "AWT-
EventQueue-0" java.lang.NullPointerException", where are the
problems???
Thank you to all....