#3450349 Prevent type error

Closes #3450349

mailchimp_transactional.module

@@ -49,6 +49,10 @@ function mailchimp_transactional_get_module_key_names() {
 
   if (!empty($modules)) {
     foreach (array_keys($modules) as $module_name) {
+      if (!isset($descriptions[$module_name])) {
+        continue;
+      }
+
       foreach ($modules[$module_name] as $key => $options) {
         $id = $module_name . '_' . $key;