error java.lang.NullPointerException [duplicate]

I have this error and I can not find the problem.

  

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException     at windows.WebPage.getUserList (MainPage.java:53) at   windows.TelePrincipal.Show_Users_In_JTable (MainPage.java:69) at   windows.HomePrincipal. (MainPage.java:28) at   windows.Tap.main.lambda $ main $ 0 (Main.java:256) at   java.awt.event.InvocationEvent.dispatch (InvocationEvent.java:311) at   java.awt.EventQueue.dispatchEventImpl (EventQueue.java:756) at   java.awt.EventQueue.access $ 500 (EventQueue.java:97) at   java.awt.EventQueue $ 3.run (EventQueue.java:709) at   java.awt.EventQueue $ 3.run (EventQueue.java:703) at   java.security.AccessController.doPrivileged (Native Method) at   java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege (ProtectionDomain.java:76)     at java.awt.EventQueue.dispatchEvent (EventQueue.java:726) at   java.awt.EventDispatchThread.pumpOneEventForFilters (EventDispatchThread.java:201)     at   java.awt.EventDispatchThread.pumpEventsForFilter (EventDispatchThread.java:116)     at   java.awt.EventDispatchThread.pumpEventsForHierarchy (EventDispatchThread.java:105)     at   java.awt.EventDispatchThread.pumpEvents (EventDispatchThread.java:101)     at   java.awt.EventDispatchThread.pumpEvents (EventDispatchThread.java:93)     at java.awt.EventDispatchThread.run (EventDispatchThread.java:82)

Main Class

/*
* To change this license header, choose License Headers in Project Properties.
* To change this template file, choose Tools | Templates
* and open the template in the editor.
*/
package janelas;

import classes.User;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;
import javax.swing.table.DefaultTableModel;

/**
*
* @author danilo
*/
public final class TelaPrincipal extends javax.swing.JFrame {

    /**
    * Creates new form TelaPrincipal
    */
    public TelaPrincipal() {
        initComponents();
        Show_Users_In_JTable();
    }

    public Connection getConnection(){

        Connection con;
        try{
            Class.forName("com.mysql.jdbc.Driver");
            con = DriverManager.getConnection("jdbc:mysql://localhost:3306/sistema_clientes","danilo","2701232");
            return con;
        } catch (ClassNotFoundException | SQLException e){
            return null;
        }

    }

    public ArrayList<User> getUserList(){

        ArrayList<User> usersList = new ArrayList<>();
        Connection connection = getConnection();
        String query = "SELECT * FROM 'users'";
        Statement st;
        ResultSet rs;

        try{
            st = connection.createStatement();
            rs = st.executeQuery(query);
            User user;
            while(rs.next()){
                user = new User(rs.getInt("id"),rs.getString("first_name"),rs.getString("last_name"),rs.getInt("age"));
                usersList.add(user);
            }
        } catch(SQLException e){
        }
        return usersList;
    }

    // Display Data in JTable
    public void Show_Users_In_JTable(){

        ArrayList<User> list = getUserList();
        DefaultTableModel model = (DefaultTableModel)jTable_Display_Users.getModel();
        Object[] row = new Object[4];

        for(int i = 0; i < list.size();i++){
            row[0] = list.get(i).getId();
            row[1] = list.get(i).getFirstName();
            row[2] = list.get(i).getLastName();
            row[3] = list.get(i).getAge();

            model.addRow(row);
        }

    }

    /**
    * This method is called from within the constructor to initialize the form.
    * WARNING: Do NOT modify this code. The content of this method is always
    * regenerated by the Form Editor.
    */
    @SuppressWarnings("unchecked")
    private void initComponents() {

        jPanel1 = new javax.swing.JPanel();
        jScrollPane1 = new javax.swing.JScrollPane();
        jTable_Display_Users = new javax.swing.JTable();
        jLabel1 = new javax.swing.JLabel();
        jTextField_Id = new javax.swing.JTextField();
        jLabel2 = new javax.swing.JLabel();
        jTextField_FirstName = new javax.swing.JTextField();
        jLabel3 = new javax.swing.JLabel();
        jTextField_LastName = new javax.swing.JTextField();
        jLabel4 = new javax.swing.JLabel();
        jTextField_Age = new javax.swing.JTextField();
        jButton_Insert = new javax.swing.JButton();
        jButton_Update = new javax.swing.JButton();
        jButton_Delete = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jTable_Display_Users.setModel(new javax.swing.table.DefaultTableModel(
            new Object [][] {

            },
            new String [] {
                "Id", "First Name", "Last Name", "Age"
            }
        ));
        jScrollPane1.setViewportView(jTable_Display_Users);

        javax.swing.GroupLayout jPanel1Layout = new javax.swing.GroupLayout(jPanel1);
        jPanel1.setLayout(jPanel1Layout);
        jPanel1Layout.setHorizontalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, jPanel1Layout.createSequentialGroup()
                .addContainerGap(153, Short.MAX_VALUE)
                .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(35, 35, 35))
        );
        jPanel1Layout.setVerticalGroup(
            jPanel1Layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(jPanel1Layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jScrollPane1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap(52, Short.MAX_VALUE))
        );

        jLabel1.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel1.setText("Id:");

        jTextField_Id.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jLabel2.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel2.setText("First Name:");

        jTextField_FirstName.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jLabel3.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel3.setText("Last Name:");

        jTextField_LastName.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jLabel4.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N
        jLabel4.setText("Age:");

        jTextField_Age.setFont(new java.awt.Font("Dialog", 0, 18)); // NOI18N

        jButton_Insert.setText("Insert");

        jButton_Update.setText("Update");

        jButton_Delete.setText("Delete");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.TRAILING)
                            .addComponent(jLabel1)
                            .addComponent(jLabel4)
                            .addComponent(jLabel3)
                            .addComponent(jLabel2))
                        .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                        .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                            .addComponent(jTextField_Id, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jTextField_FirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jTextField_LastName, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE)
                            .addComponent(jTextField_Age, javax.swing.GroupLayout.PREFERRED_SIZE, 142, javax.swing.GroupLayout.PREFERRED_SIZE))
                        .addGap(0, 1, Short.MAX_VALUE))
                    .addGroup(layout.createSequentialGroup()
                        .addGap(0, 0, Short.MAX_VALUE)
                        .addComponent(jButton_Insert)
                        .addGap(18, 18, 18)
                        .addComponent(jButton_Update)
                        .addGap(18, 18, 18)
                        .addComponent(jButton_Delete)))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addComponent(jPanel1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                .addGap(0, 0, Short.MAX_VALUE))
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel1)
                    .addComponent(jTextField_Id, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel2)
                    .addComponent(jTextField_FirstName, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel3)
                    .addComponent(jTextField_LastName, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.UNRELATED)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel4)
                    .addComponent(jTextField_Age, javax.swing.GroupLayout.PREFERRED_SIZE, 31, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addGap(102, 102, 102)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jButton_Insert)
                    .addComponent(jButton_Update)
                    .addComponent(jButton_Delete))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );

        pack();
    }

    /**
    * @param args the command line arguments
    */
    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
        * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
        */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(TelaPrincipal.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(() -> {
            new TelaPrincipal().setVisible(true);
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton_Delete;
    private javax.swing.JButton jButton_Insert;
    private javax.swing.JButton jButton_Update;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JLabel jLabel4;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JScrollPane jScrollPane1;
    private javax.swing.JTable jTable_Display_Users;
    private javax.swing.JTextField jTextField_Age;
    private javax.swing.JTextField jTextField_FirstName;
    private javax.swing.JTextField jTextField_Id;
    private javax.swing.JTextField jTextField_LastName;
    // End of variables declaration                   
}

User class

package classes;

public class User{

    //Atributos
    private int id,age;
    private String firstName, lastName;

    //Métodos Especiais
    public User(int id,String firstName, String lastName, int age) {
        this.id = id;
        this.age = age;
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public int getId() {
        return id;
    }

    public void setId(int id) {
        this.id = id;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
}