Issue #2976468 by zenimagine: The 'to' field is being shown on private message threads

src/Form/PrivateMessageForm.php

@@ -175,7 +175,7 @@ class PrivateMessageForm extends ContentEntityForm {
   /**
    * {@inheritdoc}
    */
-  public function buildForm(array $form, FormStateInterface $form_state) {
+   public function buildForm(array $form, FormStateInterface $form_state, $privateMessageThread = NULL) {
     $form = parent::buildForm($form, $form_state);
 
     if ($privateMessageThread) {