Rada nr 1 nie używaj generowanego GUI.
Rada nr 2 Jak już musisz to sprawdzaj co generuje netbeans, aż sobie sprawdziłem :P
zamień tutaj jComboBox1 = new javax.swing.JComboBox();
na swoją deklaracje
private void initComponents() {
** jComboBox1 = new javax.swing.JComboBox();**
setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
jComboBox1.setModel(new javax.swing.DefaultComboBoxModel(new String[] { "Item 1", "Item 2", "Item 3", "Item 4" }));
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(114, 114, 114)
.addComponent(jComboBox1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addContainerGap(230, Short.MAX_VALUE))
);
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(layout.createSequentialGroup()
.addGap(128, 128, 128)
.addComponent(jComboBox1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
.addContainerGap(152, Short.MAX_VALUE))
);
pack();
}// </editor-fold>
// Variables declaration - do not modify
private javax.swing.JComboBox jComboBox1;// **No i to zmodyfikuj **
// End of variables declaration