Issue #2257757 by martin107: \Drupal::isConfigSyncing should be marked as static.

core/lib/Drupal.php

@@ -618,7 +618,7 @@ public static function formBuilder() {
    * @return bool
    *   Returns TRUE is syncing flag set.
    */
-  public function isConfigSyncing() {
+  public static function isConfigSyncing() {
     return static::$container->get('config.installer')->isSyncing();
   }