Error "An illegal choice has been detected. Please contact the site administrator." [duplicate]

Question

I get the error shown below while trying to implement dependent dropdown for country state as shown below
while trying to submit form which implements AJAX in drupal 8 for country - state dropdown I get the error
Error "An illegal choice has been detected. Please contact the site
administrator.". How can I make sure default value of State is retained like its retained for Country ?< br>

namespace Drupal\custom_changes\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\custom_changes\NameTable\NameTable;
use CommerceGuys\Addressing\Subdivision\SubdivisionRepository;

/**
 * Simple Form
 * 
 */
class CustomDicForm extends FormBase {
  /**
   * @inheritdoc
   */
  public function buildForm(array $form, FormStateInterface $form_state )
  {

    $form['country'] = [
      '#type' => 'select',
      '#title' => $this->t('Country'),
      '#options' => [
        '' => t('Select'),
        'IN' => t('India'),
        'GB' => t('UK (Great Britain)'),
      ],
      '#ajax' => [
        'callback' => '::stateCallback',
        'event' => 'change',
        'wrapper' => 'state-list',
        'progress' => [
          'type' => 'throbber',
          'message' => $this->t('Updating States...'),
        ],
      ],
    ];
    if (isset($form['#options'])) {
      $form['state'] = [
        '#type' => 'select',
        '#prefix' => '<div id="state-list">',
        '#suffix' => '</div>',
        '#title' => $this->t('State'),
      ];
    }
    else {
      $form['state'] = [
        '#type' => 'select',
        '#prefix' => '<div id="state-list">',
        '#suffix' => '</div>',
        '#title' => $this->t('State'),
        '#options' => ['' => t('Select')],
      ];
    }
    $form['submit'] = [
      '#type'=> 'submit',
      '#value' => t('Submit'),
    ];

    return $form;
  }
  public function stateCallback(array &$form, FormStateInterface $form_state) {
     $country = $form_state->getValue('country');
     $form_state->setValue('state', '');
     $form['state']['#options'] = ['' => t('Select')] + $this->getAllStates(
       $country
     );

     return $form['state'];
  }

  /**
   * 
   *  Get the State from Country
   *  @return array $states
   *
   */

  public function getAllStates($country) {
    if ($country == "GB") {
      // as GB does not give values so adding hard coded values
      return ['A','B','C','D','E','F'];
    }
    else {
      $subdivisionRepository = new SubdivisionRepository();
      // Get the subdivisions for Brazil.
      $states = $subdivisionRepository->getAll([$country]);
      $in_states = array();
      foreach ($states as $key => $state) {
        $in_states[] = $key;
      }
    }
    return $in_states;
  }

  /**
   * @inheritdoc
   */
  public function getFormId()
  {
    return 'custom_changes_dic_name_form';
  }

  /**
   * @inheritdoc
   */
  public function submitForm(array &$form, FormStateInterface $form_state)
  {
    parent::submitForm($form, $form_state);
    $form['country']['#default_value'] = $form_state->getValue('country');
    $form['state']['#options'] = ['' => 'Select'] + $this->getAllStates($form['country']['#default_value']);
    $form['state']['#default_value'] = $form_state->getValue('state');
    $first_name = $form_state->getValue('first_name');  
    $last_name = $form_state->getValue('last_name');
    //@todo check how to pass exception
    $this->nameTable->insertData($first_name, $last_name);  
  }

}