diff --git a/modules/views_rss_core/views_rss_core.inc b/modules/views_rss_core/views_rss_core.inc
index 0f37c7b..24fcfc2 100644
--- a/modules/views_rss_core/views_rss_core.inc
+++ b/modules/views_rss_core/views_rss_core.inc
@@ -5,7 +5,7 @@
  * Preprocess functions for Views RSS: Core Elements module.
  */
 
-use Drupal\Component\Utility\String;
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Component\Utility\UrlHelper;
 use Drupal\taxonomy\Entity\Term;
 
@@ -25,7 +25,7 @@ function views_rss_core_preprocess_channel_title(&$variables) {
     $title = $variables['view']->getTitle();
   }
 
-  $variables['elements'][0]['value'] = String::checkPlain($title);
+  $variables['elements'][0]['value'] = SafeMarkup::checkPlain($title);
 }
 
 /**
@@ -68,7 +68,7 @@ function views_rss_core_preprocess_channel_atom_link(&$variables) {
  */
 function views_rss_core_preprocess_channel_language(&$variables) {
   if (empty($variables['elements'][0]['value'])) {
-    $variables['elements'][0]['value'] = String::checkPlain(\Drupal::languageManager()->getCurrentLanguage()->getId());
+    $variables['elements'][0]['value'] = SafeMarkup::checkPlain(\Drupal::languageManager()->getCurrentLanguage()->getId());
   }
 }
 
@@ -360,6 +360,6 @@ function views_rss_core_preprocess_item_source(&$variables) {
     $url_options['query'] = $input;
   }
 
-  $variables['elements'][0]['value'] = String::checkPlain($title);
+  $variables['elements'][0]['value'] = SafeMarkup::checkPlain($title);
   $variables['elements'][0]['attributes'] = array('url' => _url($variables['view']->getUrl(), $url_options));
 }
diff --git a/modules/views_rss_core/views_rss_core.module b/modules/views_rss_core/views_rss_core.module
index bc0ca4f..deaa2b4 100644
--- a/modules/views_rss_core/views_rss_core.module
+++ b/modules/views_rss_core/views_rss_core.module
@@ -6,6 +6,7 @@
  */
 
 use Drupal\Core\Url;
+use Drupal\Component\Utility\UrlHelper;
 
 /**
  * Include file with all preprocess functions.
@@ -255,32 +256,33 @@ function views_rss_core_views_query_alter($view, $query) {
 /**
  * Implements hook_views_rss_options_form_validate().
  */
-function views_rss_core_views_rss_options_form_validate($form, &$form_state) {
+function views_rss_core_views_rss_options_form_validate($form, $form_state) {
+  $form_state_values = $form_state->getValues();
 
   // Validate channel <image> element.
-  if (!empty($form_state['values']['style_options']['channel']['core']['views_rss_core']['image'])) {
+  if (!empty($form_state_values['style_options']['channel']['core']['views_rss_core']['image'])) {
     // Do not validate absolute URLs, as this could mean external image.
-    if (!valid_url($form_state['values']['style_options']['channel']['core']['views_rss_core']['image'], TRUE)) {
+    if (!UrlHelper::isValid($form_state_values['style_options']['channel']['core']['views_rss_core']['image'], TRUE)) {
       // Check that image exists and is in acceptable format.
-      $image = image_load($form_state['values']['style_options']['channel']['core']['views_rss_core']['image']);
-      if (empty($image)) {
-        form_set_error('style_options][channel][core][views_rss_core][image', t('Unable to find %image or incorrect image format.', array(
-          '%image' => $form_state['values']['style_options']['channel']['core']['views_rss_core']['image'],
+      $image =  \Drupal::service('image.factory')->get($form_state_values['style_options']['channel']['core']['views_rss_core']['image']);
+      if (!$image->isValid()) {
+        $form_state->setErrorByName('style_options][channel][core][views_rss_core][image', t('Unable to find %image or incorrect image format.', array(
+          '%image' => $form_state_values['style_options']['channel']['core']['views_rss_core']['image'],
         )));
       }
       else {
         // Check image width.
-        if ($image->info['width'] > 144) {
-          form_set_error('style_options][channel][core][views_rss_core][image', t("Maximum allowed width of an image for feed channel's &lt;image&gt; element is 144 pixels. Specified %image is !width pixels wide.", array(
-            '%image' => $form_state['values']['style_options']['channel']['core']['views_rss_core']['image'],
-            '!width' => $image->info['width'],
+        if ($image->getWidth() > 144) {
+          $form_state->setErrorByName('style_options][channel][core][views_rss_core][image', t("Maximum allowed width of an image for feed channel's &lt;image&gt; element is 144 pixels. Specified %image is !width pixels wide.", array(
+            '%image' => $form_state_values['style_options']['channel']['core']['views_rss_core']['image'],
+            '%width' => $image->getWidth(),
           )));
         }
         // Check image height.
         if ($image->info['height'] > 400) {
-          form_set_error('style_options][channel][core][views_rss_core][image', t("Maximum allowed height of an image for feed channel's &lt;image&gt; element is 400 pixels. Specified %image is !height pixels high.", array(
-            '%image' => $form_state['values']['style_options']['channel']['core']['views_rss_core']['image'],
-            '!height' => $image->info['height'],
+          $form_state->setErrorByName('style_options][channel][core][views_rss_core][image', t("Maximum allowed height of an image for feed channel's &lt;image&gt; element is 400 pixels. Specified %image is !height pixels high.", array(
+            '%image' => $form_state_values['style_options']['channel']['core']['views_rss_core']['image'],
+            '%height' => $image->getHeight(),
           )));
         }
       }
@@ -288,9 +290,9 @@ function views_rss_core_views_rss_options_form_validate($form, &$form_state) {
   }
 
   // Validate channel <docs> element.
-  if (!empty($form_state['values']['style_options']['channel']['core']['views_rss_core']['docs'])) {
-    if (!valid_url($form_state['values']['style_options']['channel']['core']['views_rss_core']['docs'], TRUE)) {
-      form_set_error('style_options][channel][core][views_rss_core][docs', t("Not a valid URL."));
+  if (!empty($form_state_values['style_options']['channel']['core']['views_rss_core']['docs'])) {
+    if (!UrlHelper::isValid($form_state_values['style_options']['channel']['core']['views_rss_core']['docs'], TRUE)) {
+      $form_state->setErrorByName('style_options][channel][core][views_rss_core][docs', t("Not a valid URL."));
     }
   }
 }
diff --git a/modules/views_rss_format/views_rss_format.module b/modules/views_rss_format/views_rss_format.module
index dcf268c..1d4437e 100644
--- a/modules/views_rss_format/views_rss_format.module
+++ b/modules/views_rss_format/views_rss_format.module
@@ -7,7 +7,6 @@
  * template_preprocess_views_view_row_rss() implementation.
  */
 
-use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Template\Attribute;
 
@@ -42,7 +41,7 @@ function views_rss_format_xml_elements($array) {
         }
 
         if (isset($value['value']) && $value['value'] != '') {
-          $output .= '>' . (is_array($value['value']) ? views_rss_format_xml_elements($value['value']) : (!empty($value['encoded']) ? $value['value'] : String::checkPlain($value['value']))) . '</' . $value['key'] . ">\n";
+          $output .= '>' . (is_array($value['value']) ? views_rss_format_xml_elements($value['value']) : (!empty($value['encoded']) ? $value['value'] : SafeMarkup::checkPlain($value['value']))) . '</' . $value['key'] . ">\n";
         }
         else {
           $output .= " />\n";
@@ -50,7 +49,7 @@ function views_rss_format_xml_elements($array) {
       }
     }
     else {
-      $output .= ' <' . $key . '>' . (is_array($value) ? views_rss_format_xml_elements($value) : String::checkPlain($value)) . "</$key>\n";
+      $output .= ' <' . $key . '>' . (is_array($value) ? views_rss_format_xml_elements($value) : SafeMarkup::checkPlain($value)) . "</$key>\n";
     }
   }
   // @todo This is marking the output string as safe HTML, but we have only
@@ -79,8 +78,8 @@ function views_rss_format_xml_elements($array) {
 function views_rss_format_preprocess_views_view_row_rss(&$variables) {
   $item = $variables['row'];
 
-  $variables['title'] = String::checkPlain($item->title);
+  $variables['title'] = SafeMarkup::checkPlain($item->title);
   $variables['link'] = check_url($item->link);
-  $variables['description'] = String::checkPlain($item->description);
+  $variables['description'] = SafeMarkup::checkPlain($item->description);
   $variables['item_elements'] = empty($item->elements) ? '' : views_rss_format_xml_elements($item->elements);
 }