JPanel doesn't update when adding Component in another class

I'm fairly new to Java Swing and I'm running into a few problems.

1. As a side question, when making a fairly large Java Swing Application, what is the best way to split up code? In my case I want to have an application that has a layout just as Microsoft Word where there is a JToolBar filled with buttons and a main JPanel where changes are made based on the buttons pressed in the Tool Bar.

2. So as shown in the code below, I have a JFrame and I call the MainPanel class in order to create a panel and add a ToolBar with a button. When the button is pressed it adds a button to the panel. The problem comes when you click the button nothing shows up until you resize the window(in my case I simply manually drag the screen to make it larger).

   public class Main {
   
   private static void createAndShowGUI() {
       //Create and set up the window.
       JFrame frame = new JFrame("MathMaker");
       frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
   
       //Create the menu bar.  Make it have a green background.
       //MainToolBar mainTB = new MainToolBar();
       MainPanel mainPanel = new MainPanel();
   
       frame.getContentPane().add(mainPanel.getGUI(), BorderLayout.CENTER);
       frame.pack();
       frame.setVisible(true);
   }
   
   public static void main(String[] args) {
       //Schedule a job for the event-dispatching thread:
       //creating and showing this application's GUI.
       javax.swing.SwingUtilities.invokeLater(new Runnable() {
           public void run() {
               createAndShowGUI();
           }
       });
   }
   
   }
   

   public class MainPanel implements ActionListener{
   JPanel mPanel;
   JToolBar mToolBar;
   JButton addQuestion;
       public MainPanel() {
           mPanel = new JPanel(new BorderLayout());
           mToolBar = new JToolBar();
           addQuestion = new JButton("test");
   
   
       addQuestion.addActionListener(this);
   
       mPanel.setLayout(new BorderLayout());
       mPanel.setBackground(new Color(248, 213, 131));
       mPanel.setPreferredSize(new Dimension(200, 180));
   
       mToolBar.add(addQuestion);
       mPanel.add(mToolBar, BorderLayout.PAGE_START);
   }
   public JComponent getGUI()
   {
       return mPanel;
   }
   
   @Override
   public void actionPerformed(ActionEvent e) {
        JButton temp = new JButton("temp");
        mPanel.add(temp);
   }
   

   }

You should revalidate your panel

@Override
public void actionPerformed(ActionEvent e) {
   JButton temp = new JButton("temp");
   mPanel.add(temp);
   mPanel.revalidate();
   mPanel.repaint();
}