Java.Lang.NullPoinerException when trying to insert database data into a JComboBox [duplicate]

Today my team and I encountered a problem when trying to do something seemingly simple: display the surname of an official in a JComboBox. The problem is that at the time of opening the screen we receive a NullPointerException and the Combo is empty. This is how we did it:

 private void AddFunconarioCombo () throws Exception
 {
     PreparedStatement ps = null;
     ResultSet rs = null;
try{



        String sql = "select nome_funcionario, sobrenome_funcionario from tbl_funcionario where flag_ativo =1";


        ps = this.con.prepareStatement(sql);
        rs = ps.executeQuery();
        while(rs.next())
        {          
            ComboNomeUsuario.removeAllItems();
            ComboNomeUsuario.addItem(rs.getString("nome_funcionario") + " " + rs.getString("sobrenome_funcionario"));


        }



    } catch (Exception e) {
        throw new Exception("Erro ao buscar os dados para o cadastro de Usuários!  " + e.toString());
    }
    finally
{
    ConnectionFactory.CloseConnection(con, ps, rs);
}



 }

The above method makes a database query that returns the data registered in the tbl_funcionario table, and then inserts into the ComboBox ComboNameUsuario, then I call this method in the class constructor and in the Actionlistener of a button. Here's the error stack:

  

Feb 23, 2018 4:24:31 PM view.CatalogueTelevision   SERIOUS: null   java.lang.Exception: Error fetching the data for the user registry! java.lang.NullPointerException       at view.CadastroTree.AddFunconarioCombo (Cadastro.java:1633)       at view.Catalogue Screen.       at view.Relay $ 7.actionPerformed (Rearguard.java:399)       at javax.swing.AbstractButton.fireActionPerformed (AbstractButton.java:2022)       at javax.swing.AbstractButton $ Handler.actionPerformed (AbstractButton.java:2348)       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.Component.processMouseEvent (Component.java:6533)       at javax.swing.JComponent.processMouseEvent (JComponent.java:3324)       at java.awt.Component.processEvent (Component.java:6298)       at java.awt.Container.processEvent (Container.java:2236)       at java.awt.Component.dispatchEventImpl (Component.java:4889)       at java.awt.Container.dispatchEventImpl (Container.java:2294)       at java.awt.Component.dispatchEvent (Component.java:4711)       at java.awt.LightweightDispatcher.retargetMouseEvent (Container.java:4888)       at java.awt.LightweightDispatcher.processMouseEvent (Container.java:4525)       at java.awt.LightweightDispatcher.dispatchEvent (Container.java:4466)       at java.awt.Container.dispatchEventImpl (Container.java:2280)       at java.awt.Window.dispatchEventImpl (Window.java:2746)       at java.awt.Component.dispatchEvent (Component.java:4711)       at java.awt.EventQueue.dispatchEventImpl (EventQueue.java:758)       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:80)       at java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege (ProtectionDomain.java:90)       at java.awt.EventQueue $ 4.run (EventQueue.java:731)       at java.awt.EventQueue $ 4.run (EventQueue.java:729)       at java.security.AccessController.doPrivileged (Native Method)       at java.security.ProtectionDomain $ JavaSecurityAccessImpl.doIntersectionPrivilege (ProtectionDomain.java:80)       at java.awt.EventQueue.dispatchEvent (EventQueue.java:728)       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)

And after that, how do I use the data from this combobox and insert it into the database? Follow the DAO Code

public void Salvar (Usuario u) throws Exception{

    PreparedStatement ps = null;

    if (u == null)
    {
        throw new Exception("Erro: Usuario não pode ser nulo!");
    }

    try {


        String sql = "insert into tbl_usuario (id_usuario, login_usuario, senha_usuario, fk_funcionario, flag_ativo)"
                + "values (NEXTVAL('sequencia_usuario'),?,?,CURRVAL('sequencia_funcionario'),1)";

        ps = this.con.prepareStatement(sql);
        ps.setString(1, u.getLoginUsuario());
        ps.setString(2, u.getSenhaUsuario());


        ps.executeUpdate();

        ps.close();

    } catch (Exception e) {
        throw new Exception("Erro ao inserir os dados!" + e.getMessage());
    }
    finally{
        ConnectionFactory.CloseConnection(con, ps);
    }

}

Thanks in advance for your attention.