Erro inserir no banco MySQL

package utiliarios;

import java.sql.*;
import javax.swing.JOptionPane;

public class Conector {
      final private String driver = "com.mysql.jdbc.Driver";
      final private String dbUrl="jdbc:mysql://localhost:3306/test";
      final private String user="root";
      final private String pwd="123456";
      private Connection conexao;
      public Statement statement;
      public ResultSet resultset;
      
      public boolean conecta()
      {      
          boolean resul = true;
          try
          {
              Class.forName(driver);
              conexao = DriverManager.getConnection(dbUrl, user, pwd);
              JOptionPane.showMessageDialog(null,"Conectou");
          }catch(ClassNotFoundException Driver)
          {
              JOptionPane.showMessageDialog(null, "Driver não encontrado: "+Driver); 
              resul = false;
          }catch(SQLException Fonte){
              JOptionPane.showMessageDialog(null, "Erro na Conexão com o Banco: "+Fonte);
              resul = false;
          }
          return resul;
      }
      
      public void desconectar(){
          boolean resul = true;
          
          try{
              conexao.close();
              JOptionPane.showMessageDialog(null, "banco fechado");
          }catch(SQLException erroSQL){
              JOptionPane.showMessageDialog(null, "Não foi possível fechar o banco"+ erroSQL.getMessage());
              resul = false;
          } 
      }
       public void executeSQL(String sql){
          try
          {
              statement = conexao.createStatement(ResultSet.TYPE_SCROLL_SENSITIVE,ResultSet.CONCUR_UPDATABLE);
              resultset = statement.executeQuery(sql);
          }catch(SQLException sqlex)
          {
              JOptionPane.showMessageDialog(null, sqlex.getMessage()+sql);
          }
       }
}

private void jb_salvarActionPerformed(java.awt.event.ActionEvent evt) {                                          
    
        
        
        if ((tf_nome.getText().isEmpty() )){
                
        
            JOptionPane.showMessageDialog(null, "Os campos não podem retornar vazios");
        }
        else{ 
                    
       try{
           String nome=("INSERT INTO cadastro_usuario (nome) VALUES  ('"+tf_nome.getText()+"')");
           c.statement.executeUpdate(nome);
           JOptionPane.showMessageDialog(null,"Usuário cadastrado com sucesso.");
           
           
       }
       catch (SQLException erro){
           JOptionPane.showMessageDialog(null,"Erro"+erro);
       }
        }      
       
    }

Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
	at cadastros.cadastro_usuario.jb_salvarActionPerformed(cadastro_usuario.java:215)
	at cadastros.cadastro_usuario.access$200(cadastro_usuario.java:14)
	at cadastros.cadastro_usuario$3.actionPerformed(cadastro_usuario.java:93)
	at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:2018)
	at javax.swing.AbstractButton$Handler.actionPerformed(AbstractButton.java:2341)
	at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel.java:402)
	at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:259)
	at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(BasicButtonListener.java:252)
	at java.awt.AWTEventMulticaster.mouseReleased(AWTEventMulticaster.java:289)
	at java.awt.Component.processMouseEvent(Component.java:6505)
	at javax.swing.JComponent.processMouseEvent(JComponent.java:3321)
	at java.awt.Component.processEvent(Component.java:6270)
	at java.awt.Container.processEvent(Container.java:2229)
	at java.awt.Component.dispatchEventImpl(Component.java:4861)
	at java.awt.Container.dispatchEventImpl(Container.java:2287)
	at java.awt.Component.dispatchEvent(Component.java:4687)
	at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:4832)
	at java.awt.LightweightDispatcher.processMouseEvent(Container.java:4492)
	at java.awt.LightweightDispatcher.dispatchEvent(Container.java:4422)
	at java.awt.Container.dispatchEventImpl(Container.java:2273)
	at java.awt.Window.dispatchEventImpl(Window.java:2719)
	at java.awt.Component.dispatchEvent(Component.java:4687)
	at java.awt.EventQueue.dispatchEventImpl(EventQueue.java:723)
	at java.awt.EventQueue.access$200(EventQueue.java:103)
	at java.awt.EventQueue$3.run(EventQueue.java:682)
	at java.awt.EventQueue$3.run(EventQueue.java:680)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:87)
	at java.awt.EventQueue$4.run(EventQueue.java:696)
	at java.awt.EventQueue$4.run(EventQueue.java:694)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.security.ProtectionDomain$1.doIntersectionPrivilege(ProtectionDomain.java:76)
	at java.awt.EventQueue.dispatchEvent(EventQueue.java:693)
	at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:244)
	at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:163)
	at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:151)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:147)
	at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:139)
	at java.awt.EventDispatchThread.run(EventDispatchThread.java:97)