bug #9397 [BUG][Form] Fix nonexistent key id in twig of data collector (francoispluchino)

This PR was merged into the master branch.

Discussion
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[BUG][Form] Fix nonexistent key id in twig of data collector

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

When you create a single button form type, the id of data collector does not exist. It's caused by the PR #9394.

Commits
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19c74f3 Fix nonexistent key id in twig of data collector

src/Symfony/Component/Form/Extension/DataCollector/FormDataExtractor.php

@@ -123,7 +123,9 @@ class FormDataExtractor implements FormDataExtractorInterface
      */
     public function extractViewVariables(FormView $view)
     {
-        $data = array();
+        $data = array(
+            'id' => $view->vars['id']
+        );
 
         foreach ($view->vars as $varName => $value) {
             $data['view_vars'][$varName] = $this->valueExporter->exportValue($value);