Payment form callbacks must return the form element.

Author: torotil

src/Component.php

@@ -139,7 +139,7 @@ protected function methodForm($method, &$form_state) {
     $form_elements_callback = $method->controller->payment_configuration_form_elements_callback;
     if (function_exists($form_elements_callback) == TRUE) {
       // $element is changed by reference.
-      $form_elements_callback($element, $form_state, $payment);
+      $element += $form_elements_callback($element, $form_state, $payment);
     }
     return $element;
   }