How do I use and administrative form and save a message? I followed the instructions on Step by step guide to creating an admin form in Drupal 8.

I typed in a welcome text into the admin form and saved it, but after saving the form was empty again.

I am new to Drupal and php and don't know yet how to debug the code. I can not use Php-Storm because it is too expensive. Therefore I am using Visual Studio.

The website and database are running inside Docker. I mounted the folder for the modules, so I can write the module code from outside docker. I am working on a Ubuntu 18.04 machine.

This is the whole class:

namespace Drupal\welcome\Form;

use Drupal\Core\Form\ConfigFormBase;
use Drupal\Core\Form\FormStateInterface;

class MessagesForm extends ConfigFormBase {

  /**
   * {@inheritdoc}
   */
  protected function getEditableConfigNames() {
    return [
      'welcome.adminsettings',
    ];
  }

  /**
   * {@inheritdoc}
   */
  public function getFormID() {
    return 'welcome_form';
  }

  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    $config = $this-&gt;config('hello_world.adminsettings');

    $form['welcome_message'] = [
      '#type'          =&gt; 'textarea',
      '#title'         =&gt; $this-&gt;t('Willkommens Nachricht'),
      '#description'   =&gt; $this-&gt;t('Willkommens Nachricht, die beim Anmelden erscheint.'),
      '#default_value' =&gt; $config-&gt;get('welcome_message'),
    ];

    return parent::buildForm($form, $form_state);
  }

  /**
   * {@inheritdoc}
   */
  public function submitForm(array &amp;$form, FormStateInterface $form_state) {
    parent::submitForm($form, $form_state);

    $this-&gt;config('welcome.adminsettings')
      -&gt;set('welcome_message', $form_state-&gt;getValue('welcome_message'))
      -&gt;save();
  }
}


I just learned how to inpect variables with ksm() and found that the value is stored in the welcome_message variable.
Yes, I found the bug.

In buildForm is says: 

hello_world.adminsettings


But it must be: 

 welcome.adminsettings