My code contains no error but when I run my project/source code,this appears in the stack trace

'Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException' and I believe its the first thing I should work on.What does that mean and how should I solve that.Here is my simple code.

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.SQLException;


try {

String username = "username";
String password = "password";


if (rs.next()) {

if (username.isEmpty() || password == null);
{
System.out.println("Write a Correct UserName and Password");
}

if ((!(username.equals("null")) && password.equals(rs.getString("password")))
&& (!(password.equals("null")) && username.equals(rs.getString("username")))) {
System.out.println("Login Successful");

String driver = "org.apache.derby.ClientDriver";
String url = "jdbc:derby://localhost:1527/sample";
String uName = "app";
String uPass = "app";
con = DriverManager.getConnection(url, username, password);
con.close();
rs.close();

}
}
} catch (SQLException err) {
System.out.println(err.getMessage());
}
Login det = new Login ();
det.setSize(500,500);
det.setVisible(true);

}