Issue #3492110: Fix WSOD with media_library_widget

Closes #3492110

extra_field_description.module

@@ -60,22 +60,28 @@ function extra_field_description_field_widget_third_party_settings_form(WidgetIn
  * Implements hook_field_widget_single_element_form_alter().
  */
 function extra_field_description_field_widget_single_element_form_alter(array &$element, FormStateInterface $form_state, array $context) {
-  $third_party_settings = $context['widget']->getThirdPartySettings('extra_field_description', 'extra_description');
-  if (!empty($third_party_settings)) {
-    if (isset($element['value'])) {
-      if ($element['value']['#type'] == 'datetime' || $element['value']['#type'] == 'datelist') {
-        $element['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
+  $widget = $context['widget'] ?? NULL;
+  if ($widget instanceof WidgetInterface) {
+    $third_party_settings = $widget->getThirdPartySettings('extra_field_description', 'extra_description');
+    if (!empty($third_party_settings)) {
+      if (isset($element['value'])) {
+        if ($element['value']['#type'] == 'datetime' || $element['value']['#type'] == 'datelist') {
+          $element['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
+        }
+        else {
+          $element['value']['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
+        }
+      }
+      elseif (isset($element['target_id'])) {
+        $element['target_id']['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
+      }
+      elseif ('media_library_widget' === $widget->getPluginId()) {
+        $element['#field_prefix']['extra_description']['#markup'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
       }
       else {
-        $element['value']['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
+        $element['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
       }
     }
-    elseif (isset($element['target_id'])) {
-      $element['target_id']['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
-    }
-    else {
-      $element['#field_prefix'] = '<div class="extra-description" >' . $third_party_settings['extra_description']['over_description'] . '</div>';
-    }
   }
 }