Método getContentPane() AJUDA

package gui;

import java.awt.<em>;

import javax.swing.</em>;

public class JanelaBaskaraSwing extends Frame {//heranca

private JButton btCalcular;

private JTextField tfa;

private JTextField tfb;

private JTextField tfc;

private JLabel lba;

private JLabel lBb;

private JLabel LBc;

private JLabel LBx1;

private JLabel LBx2;

private JLabel LBresultx1;

private JLabel LBresultx2;

private Container container;


public JanelaBaskaraSwing() { 
    super("Formula Baskara");//metodo construtor da classe Pai com parametro.
    
    
    container = this.getContentPane(); <---- aqui ele me mostra o erro.


    container.setLayout(new FlowLayout());//definindo o formulario.
    btCalcular = new JButton("Calcular");
    tfa = new JTextField(10);
    tfb = new JTextField(10);
    tfc = new JTextField(10);
    
    
    lba = new JLabel("A:");
    lBb = new JLabel("b:");
    LBc = new JLabel("c:");
    LBx1= new JLabel ("X¹: ");
    LBx2= new JLabel ("X¹¹: ");
    LBresultx1 = new JLabel("0.0");
    LBresultx2 = new JLabel("0.0");
    
    container.add(lba);
    container.add(tfa);
    container.add(lBb);
    container.add(tfb);
    container.add(LBc);
    container.add(tfc);
    container.add(btCalcular);
    container.add(LBx1);
    container.add(LBresultx1);
    container.add(LBx2);
    container.add(LBresultx2);


    this.setSize(640,480);
    this.setVisible(true);
    
}

public static void main(String[] args) {
  
    JanelaBaskaraSwing jb = new JanelaBaskaraSwing();
}