diff --git a/src/Form/PrivateMessageForm.php b/src/Form/PrivateMessageForm.php
index d3707ab1e472f7e31e12041d2a9f68e1b05d220f..6b771a651fec51565847fc09ca92f23dec719373 100644
--- a/src/Form/PrivateMessageForm.php
+++ b/src/Form/PrivateMessageForm.php
@@ -100,11 +100,11 @@ class PrivateMessageForm extends ContentEntityForm {
   protected $formBuilder;
 
   /**
-   * The private message configuration.
+   * The message configuration.
    *
    * @var \Drupal\Core\Config\ImmutableConfig
    */
-  private ImmutableConfig $config;
+  protected ImmutableConfig $config;
 
   /**
    * Constructs a PrivateMessageForm object.
diff --git a/tests/src/Functional/PrivateMessageThreadMembersTest.php b/tests/src/Functional/PrivateMessageThreadMembersTest.php
index 2573ce1e899dd0cf24eff6d9300f72ad8c87085d..ae74cfcf52341189f8da9ad7de4f758f664aefb7 100644
--- a/tests/src/Functional/PrivateMessageThreadMembersTest.php
+++ b/tests/src/Functional/PrivateMessageThreadMembersTest.php
@@ -156,4 +156,18 @@ class PrivateMessageThreadMembersTest extends BrowserTestBase {
     $this->assertSession()->elementTextContains('css', '.private-message-recipients article:nth-of-type(2) .username', $this->userC->getDisplayName());
   }
 
+  /**
+   * Regression test for multiple add more clicks.
+   */
+  public function testMultipleAddMoreClicks(): void {
+    $this->drupalLogin($this->userA);
+
+    $this->drupalGet('/private-message/create');
+
+    $this->getSession()->getPage()->pressButton('edit-members-add-more');
+    $this->assertSession()->statusCodeEquals(200);
+    $this->getSession()->getPage()->pressButton('edit-members-add-more');
+    $this->assertSession()->statusCodeEquals(200);
+  }
+
 }