Issue #3423935: Update hook fails if content types have not been previously set

mercury_editor.install

@@ -10,7 +10,14 @@
  */
 function mercury_editor_update_9001() {
   $mercury_settings = Drupal::configFactory()->getEditable('mercury_editor.settings');
-  foreach (array_keys($mercury_settings->get('content_types')) as $content_type) {
+
+  $content_types = $mercury_settings->get('content_types');
+  
+  if (empty($content_types)) {
+    return;
+  }
+
+  foreach (array_keys($content_types) as $content_type) {
     $mercury_settings->set('bundles.node.' . $content_type, $content_type)->save();
   }
   $mercury_settings->clear('content_types')->save();