bugfix: declaration missmatch

core/DependencyInjection/Container.php

@@ -19,7 +19,7 @@ class Container extends \Illuminate\Container\Container
         return static::$instance;
     }
 
-    public static function setInstance(ContainerContract $container)
+    public static function setInstance(ContainerContract $container = null)
     {
         self::$instance = $container;
     }