CMRFDisplay: Avoid undefined array key access

Closes #3447570

modules/cmrf_form_processor_display/src/Element/CMRFDisplay.php

@@ -8,7 +8,6 @@ namespace Drupal\cmrf_form_processor_display\Element;
  * @license  GPL-2.0-or-later
  */
 
-
 use Drupal\Core\Render\Element\RenderElement;
 
 /**
@@ -41,7 +40,7 @@ class CMRFDisplay extends RenderElement {
    */
   public static function preRenderCmrfDisplay(array $element) {
     if (isset($element['#default_value'])) {
-      switch ($element['#cmrf_display_mode']) {
+      switch ($element['#cmrf_display_mode'] ?? NULL) {
         case 'link':
           $markup = t('<a target="_blank" href=":url">:text</a>.', [
             ':url' => $element['#default_value'],