Improve FormBuilderIterator with removing accessing private propertie…

…s over reflection

The property 'children' is not always accessible or even available because several implementations of other FormBuilders do not have this property.
Accessing private properties is never a good idea. Only rely on the given FormBuilderInterface.

Util/FormBuilderIterator.php

@@ -65,12 +65,7 @@ public function __construct(FormBuilderInterface $formBuilder, $prefix = false)
      */
     private static function getKeys(FormBuilderInterface $formBuilder)
     {
-        if (!self::$reflection) {
-            self::$reflection = new \ReflectionProperty(get_class($formBuilder), 'children');
-            self::$reflection->setAccessible(true);
-        }
-
-        return array_keys(self::$reflection->getValue($formBuilder));
+        return array_keys($formBuilder->all());
     }
 
     /**