Issue #3314446: Only set element value when element exists

insert.module

@@ -858,11 +858,19 @@ function _insert_allowed_html_validate(array $element, FormStateInterface &$form
 
   $additional = \Drupal::moduleHandler()->invokeAll('insert_allowed_html');
 
-  $form_state->setValueForElement($element, InsertUtility::addAllowedHtml(
-    $element['#value'],
-    NestedArray::mergeDeep($tags, isset($additional['tags']) ? $additional['tags'] : []),
-    NestedArray::mergeDeep($attributes, isset($additional['attributes']) ? $additional['attributes'] : [])
-  ));
+  if (isset($element['#value'])) {
+    $form_state->setValueForElement($element, InsertUtility::addAllowedHtml(
+      $element['#value'],
+      NestedArray::mergeDeep(
+        $tags,
+        isset($additional['tags']) ? $additional['tags'] : []
+      ),
+      NestedArray::mergeDeep(
+        $attributes,
+        isset($additional['attributes']) ? $additional['attributes'] : []
+      )
+    ));
+  }
 }
 
 /**