Error: [] operator not supported for strings in HOOK_form_alter()
>>> [!note] Migrated issue
<!-- Drupal.org comment -->
<!-- Migrated from issue #3345337. -->
Reported by: [fanton](https://www.drupal.org/user/128756)
Related to !2
>>>
<h3 id="summary-problem-motivation">Problem/Motivation</h3>
<p>When accessing to view the documentation generated for a project that includes _form_alter that adds a class, it crashes the site:</p>
<blockquote><p>The website encountered an unexpected error. Please try again later.<br>
Error: [] operator not supported for strings in MYTHEME_form_alter() (line ... of themes/custom/MYTHEME/MYTHEME.theme). </p></blockquote>
<h4 id="summary-steps-reproduce">Steps to reproduce</h4>
<p>Have a custom theme or module that uses HOOK_form_alter and adds a CSS class to the forms, for example:</p>
<pre>function MYTHEME_form_alter(&$form, &$form_state, $form_id) {<br>if (array_key_exists('#attributes', $form)) {<br> $form['#attributes']['class'][] = 'my-custom-class';<br> }<br>}</pre><h3 id="summary-proposed-resolution">Proposed resolution</h3>
<p>Change:</p>
<p><code>$form['#attributes']['class'] = 'api-search-form';</code></p>
<p>To:</p>
<p><code>$form['#attributes']['class'] = ['api-search-form'];</code></p>
<p>in SearchForm.php, buildForm function, line 34.</p>
issue