I have a simple AJAX form that I can’t submit when the form is passed from module to the template in hook_preprocess_page. The same form works fine if sent from controller, it’s just that the layout is complicated and I prefer to write the HTML part in the twig file and integrate the form where it fits best.

The form class:

    <?php  namespace DrupalmymoduleForm;  use DrupalCoreFormFormBase; use DrupalCoreFormFormStateInterface;  class SubmissionForm extends FormBase {   public function getFormId() {     return 'mymodule_submission_form';   }    public function buildForm(array $form, FormStateInterface $form_state) {     $form['#prefix'] = '<div id="submission-form-wrapper">';     $form['#suffix'] = '</div>';      $form['mail'] = array(       '#title' => 'E-mail',       '#type' => 'textfield',       '#required' => TRUE,     );      $form['actions'] = array(       '#type' => 'actions'     );      $form['actions']['submit'] = array(       '#type' => 'submit',       '#value' => 'Submit',       '#attributes' => array(         'class' => array(           'btn',           'btn-success',           'use-ajax-submit'         ),       ),       '#ajax' => array(         'wrapper' => 'submission-form-wrapper',       )     );      return $form;   }    public function validateForm(array &$form, FormStateInterface $form_state) {   }    public function submitForm(array &$form, FormStateInterface $form_state) {   } } 

The preprocess function:

/**  * Implements hook_preprocess_HOOK().  */ function mymodule_preprocess_page(&$vars) {   if (Drupal::service('path.matcher')->isFrontPage()) {         $vars['page']['submission_form'] = Drupal::formBuilder()->getForm('DrupalmymoduleFormSubmissionForm');   } } 

The error:

TypeError: Argument 1 passed to DrupalCoreAjaxAjaxResponse::setAttachments() must be of the type array, null given, called in /Volumes/Work/Git/myproject/public_html/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php on line 56 in DrupalCoreAjaxAjaxResponse->setAttachments() (line 37 of /Volumes/Work/Git/myproject/public_html/core/lib/Drupal/Core/Render/AttachmentsTrait.php).

• it’s a fresh Drupal 8.7.10
• devel and kint as contrib modules enabled
• core inline form errors enabled
• the form has only one required textfield and the submit button

Any suggestions on how to fix this?

Thank you!