Issue #3426324 by Wim Leers: ExistsConstraintValidator should ignore NULL...

Issue #3426324 by Wim Leers: ExistsConstraintValidator should ignore NULL values and treat core as a valid module

(cherry picked from commit ab695b5c)

core/modules/taxonomy/src/Plugin/views/filter/TaxonomyIndexTid.php

@@ -343,7 +343,13 @@ public function validateExposed(&$form, FormStateInterface $form_state) {
       return;
     }
 
-    $identifier = $this->options['expose']['identifier'];
+    if ($this->options['is_grouped']) {
+      $identifier = $this->options['group_info']['identifier'];
+    }
+    else {
+      $identifier = $this->options['expose']['identifier'];
+    }
+
     $input = $form_state->getValue($identifier);
 
     if ($this->options['is_grouped'] && isset($this->options['group_info']['group_items'][$input])) {