Custom submit handler not work on form alter[D8]

Hi Team,

I am currently working on Drupal 8 site in which i have alter node form using hook_form_alter() to add custom submit handler function for node.

Now problem is that my custom submit handler function is not get called on form submission. My code is below.

<?php
/**
 * @file
 * Alter node form
 */
use Drupal\Core\Form\FormStateInterface;

function drupal_to_slack_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  $type = \Drupal::config('drupal_to_slack.settings')->get('node_to_notify_on_slack');
  if (isset($type)) {
    $filter = array_filter($type);
    if (count($filter) > 1) {
      foreach ($filter as $filter_val) {
       $actual_form_id[$filter_val] = 'node_' . $filter_val . '_form';
      }
    }
    else {
      $type_var = implode('_node_form,', $filter);
      $actual_form_id = array($type_var . '_node_form' => 'node_'. $type_var . '_form');
    }
    if (in_array($form_id, $actual_form_id)) {
       $form['#validate'][] = 'drupal_to_slack_form_validate';
       $form['#submit'][][] = 'drupal_to_slack_form_submit';
    }
  }
}
/**
 *  Implement custom form submit function
 */

function drupal_to_slack_form_submit(array &$form, FormStateInterface $form_state) {
    foreach ($form_state->getValues() as $key => $value) {
        print_r( $value); // i cant catch my custom element value here T_T why?
    }
exit();    
}

Please let me know what is wrong in this code.

Thanks for help in advance.