Form/Type/LinkedCustomFieldsType.php
private function getRootForm(FormInterface $form)
    {
        if ($form->getParent() === NULL) {
            return $form;
        } else {
            return $this->getRootForm($form->getParent());
        }
    }
private function getRootForm(FormInterface $form)
    {
        if ($form->getParent() === NULL) {
            return $form;
        } else {
            return $this->getRootForm($form->getParent());
        }
    }
Return the root form (i.e. produced from CustomFieldsGroupType::getForm)

@param FormInterface $form
@return FormInterface