Suppress undefined index warnings

The patch at #6 was removing my theme suggestions when #form_id was not set. With this commit both #form_id and #type suggestions are added only if they exist.

themable_forms.module

@@ -12,12 +12,16 @@ use Drupal\Core\Render\Element;
  * Implements hook_theme_suggestions_form_element().
  */
 function themable_forms_theme_suggestions_form_element(array $variables) {
-  $suggestions = ['form_element__type__' . $variables['element']['#type']];
+  $suggestions = [];
+  if (isset($variables['element']['#type'])) {
+    $suggestions = ['form_element__type__' . $variables['element']['#type']];
+  }
   if (isset($variables['element']['#form_id'])) {
     $suggestions[] = 'form_element__form_id__' . $variables['element']['#form_id'];
+  }
+  if (isset($variables['element']['#type']) && isset($variables['element']['#form_id'])) {
     $suggestions[] = 'form_element__' . $variables['element']['#form_id'] . '__' . $variables['element']['#type'];
   }
-
   return $suggestions;
 }