Symfony version(s) affected: 4.1 and 4.2 (have not tried with other versions).

Description

This happens when I use multiple Twig loaders and run ./bin/console debug:twig. The error message is

In DebugCommand.php line 249:

   Attempted to call an undefined method named "getNamespaces" of class "Twig_Loader_Chain".

How to reproduce

• Start a new Symfony project composer create-project symfony/website-skeleton test
• Add a new Twig loader to services.yaml, under services: section, as extra_twig_loader: { class: Twig\Loader\FilesystemLoader }
• Run ./bin/console debug:twig

I might have been missing something when adding a new Twig loader, but for now, it's the error I get. Looking at DebugCommand.php code, the Twig loader instance for multiple Twig loaders is ChainLoader instead of FilesystemLoader. This ChainLoader doesn't have getNamespaces() method.