How to return the value of a sql database combobox in a Jtextfield?

Question

In my program I have a register of customers in which contain two combobox.
One is districts and the other is counties.

As I show in the next photo in the comboDistricts has several districts of Portugal.

When selecting Porto or any other district will show in the comboConcelho the counties corresponding to these districts. So the comboConcelho depends on the comboDistrito.

For example, if you choose the "Porto" list in the municipalities it will show all the municipalities of Porto as shown in the following image:

When registering a client in the database, the district and county fields show the ID corresponding to each of them. In the following photo I show an example of a registered customer with district PORTO and county MAIA

Problem is when I try to find this client, in this case loading the client into the corresponding Jtextfield returns me an error. In the next photo shows to load all fields except combos.

The error that returns me is as follows:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException at Programa.M_Clientes$2.itemStateChanged(M_Clientes.java:444) at javax.swing.JComboBox.fireItemStateChanged(Unknown Source) at javax.swing.JComboBox.selectedItemChanged(Unknown Source) at javax.swing.JComboBox.contentsChanged(Unknown Source) at javax.swing.AbstractListModel.fireContentsChanged(Unknown Source) at javax.swing.DefaultComboBoxModel.setSelectedItem(Unknown Source) at javax.swing.JComboBox.setSelectedItem(Unknown Source) at javax.swing.JComboBox.setSelectedIndex(Unknown Source) at Programa.M_Clientes.procura_contribuinte_e_coloca(M_Clientes.java:382) at Programa.M_Clientes$10.actionPerformed(M_Clientes.java:814) at javax.swing.AbstractButton.fireActionPerformed(Unknown Source) at javax.swing.AbstractButton$Handler.actionPerformed(Unknown Source) at javax.swing.DefaultButtonModel.fireActionPerformed(Unknown Source) at javax.swing.DefaultButtonModel.setPressed(Unknown Source) at javax.swing.plaf.basic.BasicButtonListener.mouseReleased(Unknown Source) at java.awt.AWTEventMulticaster.mouseReleased(Unknown Source) at java.awt.Component.processMouseEvent(Unknown Source) at javax.swing.JComponent.processMouseEvent(Unknown Source) at java.awt.Component.processEvent(Unknown Source) at java.awt.Container.processEvent(Unknown Source) at java.awt.Component.dispatchEventImpl(Unknown Source) at java.awt.Container.dispatchEventImpl(Unknown Source) at java.awt.Component.dispatchEvent(Unknown Source) at java.awt.LightweightDispatcher.retargetMouseEvent(Unknown Source) at java.awt.LightweightDispatcher.processMouseEvent(Unknown Source) at java.awt.LightweightDispatcher.dispatchEvent(Unknown Source) at java.awt.Container.dispatchEventImpl(Unknown Source) at java.awt.Window.dispatchEventImpl(Unknown Source) at java.awt.Component.dispatchEvent(Unknown Source) at java.awt.EventQueue.dispatchEventImpl(Unknown Source) at java.awt.EventQueue.access$500(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.awt.EventQueue$3.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) at java.awt.EventQueue$4.run(Unknown Source) at java.awt.EventQueue$4.run(Unknown Source) at java.security.AccessController.doPrivileged(Native Method) at java.security.ProtectionDomain$JavaSecurityAccessImpl.doIntersectionPrivilege(Unknown Source) at java.awt.EventQueue.dispatchEvent(Unknown Source) at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source) at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.pumpEvents(Unknown Source) at java.awt.EventDispatchThread.run(Unknown Source)

The photo error is also available here for those who prefer:

In the first line of error "M_clients.java.444" when clicking directs me to this code:

Code of the photo here:

comboModel1 = new DefaultComboBoxModel();
    comboModel1 = carrega_combo_distrito();
    comboBoxDistrito = new JComboBox(comboModel1);
    comboBoxDistrito.addItemListener(new ItemListener() {
        public void itemStateChanged(ItemEvent arg0) {

            comboModel2 = new DefaultComboBoxModel();
            QueryResultDistritos temp = (QueryResultDistritos) comboBoxDistrito.getSelectedItem();
            comboModel2 = carrega_combo_concelho(temp.recordCode);
            comboBoxConcelhos.setModel(comboModel2);


        }
    });

Another error line that says "M_clients.java:382" if you click directs me to this code:

Code of the photo here:

void procura_contribuinte_e_coloca(){



        ClientesLigaBD cl = new ClientesLigaBD();
        if((cl.verifica_se_existe(txt_contribuinte.getText()))){

            Clientes lido = new Clientes();

            lido = cl.LerRegisto(txt_contribuinte.getText());

            txt_nome.setText(lido.getNome_cliente());
            txt_n_cliente.setText(String.valueOf(lido.getN_cliente()));
            txt_rua_av.setText(lido.getRua_av());
            txt_n_rua_av.setText(String.valueOf(lido.getN_rua_av()));
            txt_local.setText(lido.getLocal().trim());
            txt_email.setText(lido.getEmail());
            txt_telefone.setText(String.valueOf(lido.getTelefone()));
            txt_telemovel.setText(String.valueOf(lido.getTelemovel()));
            txt_cod_postal.setText(lido.getCod_postal());

            int idxD = getComboIndexD(comboBoxDistrito, lido.getDistrito());
            comboBoxDistrito.setSelectedIndex(idxD);

            int idxC = getComboIndexC(comboBoxConcelhos, lido.getConcelho());
            comboBoxConcelhos.setSelectedIndex(idxC);






            JOptionPane.showMessageDialog(null, "Cliente encontrado !");


        }else{

            JOptionPane.showMessageDialog(null, "Não existe nenhum cliente com esse número contribuinte.");

        }
    }

This code carries clients in Jtextfields and combos.

Can anyone tell me what’s wrong with loading the combos?