Ajax error render

I receive this error message when I try my custom form in Drupal 8.

User error: “0” is an invalid render array key in Drupal\Core\Render\Element::children()

This is the code I am using

namespace Drupal\drupalup_simple_form\Form;  use Drupal\Core\Form\FormBase; use Drupal\Core\Form\FormStateInterface; use Drupal\Core\Ajax\AjaxResponse; use Drupal\Core\Ajax\HtmlCommand;  /**  * Our simple form class.  */ class SimpleForm extends FormBase {   /**    * {@inheritdoc}    */   public function getFormId() {     return 'drupalup_simple_form';   }     /**    * {@inheritdoc}    */   public function buildForm(array $  form, FormStateInterface $  form_state) {   $  form['search'] = array(     '#type' => 'textfield',     '#title' => t('Search:'),   );   $  form['submit'] = array(     '#type' => 'submit',     '#value' => $  this->t('Submit'),     '#button_type' => 'primary',     '#ajax' => array(       'callback' => [$  this, 'submitForm'],       'event' => 'click',     ),   );   return $  form; }      /**    * {@inheritdoc}    */    public function submitForm(array &$  form, FormStateInterface $  form_state) {     $  url = 'http://example.se/api/json/1.0/searchProduct.json?query=';     $  term = $  form_state->getValue('search');     $  query_url = $  url. $  term;     $  client =  \Drupal::httpClient();     $  request = $  client->get($  query_url);     $  data = json_decode($  request->getBody(), TRUE);     $  results = [];     if (!empty($  data) && !empty($  data['items'])) {       foreach ($  data['items'] as $  item ) {         $  results[] = $  item['name'];       }     }     return $  results;      }   } ?> 

The end result should be that all results from the URL with the [searchword] are shown when you have entered the search word in the textfield and pressed submit. Does anyone know how to solve this?