Issue #3450605 by kksandr: replace get_class() with static::class

Closes #3450605

src/Element/BaseSettings.php

@@ -693,7 +693,7 @@ class BaseSettings extends FormElement {
     // That allows executeElementSubmitHandlers() to be completely certain that
     // the form has passed validation before proceeding.
     $complete_form['#validate'][] = [
-      get_class(),
+      static::class,
       'executeLibraryElementSubmitHandlers',
     ];
     $complete_form['#charts_library_settings_element_submit_attached'] = TRUE;