Added array_key_exists checking to WorkflowDefaultWidget.php

Closes #3410051

src/Plugin/Field/FieldWidget/WorkflowDefaultWidget.php

@@ -80,12 +80,18 @@ class WorkflowDefaultWidget extends WidgetBase {
       'triggering_element' => $form_state->getTriggeringElement(),
     ];
     $workflow_form = WorkflowManager::getWorkflowTransitionForm($entity, $field_name, $form_state_additions);
-
+    
     // Determine and add the attached fields.
     $attached_fields = WorkflowManager::getAttachedFields('workflow_transition', $wid);
     foreach ($attached_fields as $key => $attached_field) {
-      $element[$key] = $workflow_form[$key];
+      if (isset($workflow_form[$key])) {
+        $element[$key] = $workflow_form[$key];
+      } else {
+        $element[$key] = 'Default Value';
+      }
     }
+    
+
 
     // Option 3: use the true Element.
     // $form = $this->element($form, $form_state, $transition);