bug #12308 Ensure that Twig form themes are always a list, not a hashmap (stof)

This PR was merged into the 2.6-dev branch.

Discussion
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Ensure that Twig form themes are always a list, not a hashmap

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12305
| License       | MIT
| Doc PR        | n/a

array_unique does not reindex arrays when deleting keys.

Commits
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2a19cd8 Ensure that Twig form themes are always a list, not a hashmap

src/Symfony/Bundle/TwigBundle/DependencyInjection/Configuration.php

@@ -54,7 +54,7 @@ class Configuration implements ConfigurationInterface
                     return count($v['form']['resources']) > 0;
                 })
                 ->then(function ($v) {
-                    $v['form_themes'] = array_unique(array_merge($v['form']['resources'], $v['form_themes']));
+                    $v['form_themes'] = array_values(array_unique(array_merge($v['form']['resources'], $v['form_themes'])));
 
                     return $v;
                 })