Issue #2948891: InvalidArgumentException: Value is not a valid entity.

src/Plugin/Field/FieldWidget/CshsWidget.php

@@ -101,7 +101,9 @@ class CshsWidget extends WidgetBase {
     }
     else {
       /* @noinspection NullPointerExceptionInspection */
-      $element['target_id']['#default_value'] = $items->get($delta)->getValue();
+      if (!empty($items->getValue()[$delta]->target_id)) {
+        $element['target_id']['#default_value'] = $items->getValue()[$delta]->target_id;
+      }
     }
 
     return $element;