Issue #3417654 by bohart: Fixed fatal PHP Error: Call to a member function get() on null in Drupal\private_message\Form\PrivateMessageForm->save().

src/Form/PrivateMessageForm.php

@@ -453,6 +453,8 @@ class PrivateMessageForm extends ContentEntityForm {
    */
   public function save(array $form, FormStateInterface $formState) {
     $status = parent::save($form, $formState);
+
+    /** @var \Drupal\private_message\Entity\PrivateMessageThreadInterface $entity */
     $entity = $formState->get('pmt_entity');
 
     /** @var \Drupal\private_message\Entity\PrivateMessageThreadInterface $private_message_thread */
@@ -478,8 +480,8 @@ class PrivateMessageForm extends ContentEntityForm {
     // Add subject to thread.
     // Only if the thread subject is empty.
     if ($private_message_thread->get('subject')->isEmpty()
-      && $subject = $entity->get('subject')->value) {
-      $private_message_thread->set('subject', $subject);
+      && $entity && !$entity->get('subject')->isEmpty()) {
+      $private_message_thread->set('subject', $entity->get('subject')->value);
     }
 
       // Save the thread.