Only start the session if needed to find data in FORM_DATA (see #1471).

Description
-----------

-

Commits
-------

b96c8da Only start the session if needed to find data in FORM_DATA
4e6d7cc Fix the coding style and the unit tests.

src/Resources/contao/library/Contao/Input.php

@@ -791,6 +791,14 @@ public static function findPost($strKey)
 			return $_POST[$strKey];
 		}
 
+		$request = System::getContainer()->get('request_stack')->getMasterRequest();
+
+		// Return if the session has not been started before
+		if ($request === null || !$request->hasPreviousSession())
+		{
+			return null;
+		}
+
 		if (isset($_SESSION['FORM_DATA'][$strKey]))
 		{
 			return ($strKey == 'FORM_SUBMIT') ? preg_replace('/^auto_/i', '', $_SESSION['FORM_DATA'][$strKey]) : $_SESSION['FORM_DATA'][$strKey];

tests/Contao/WidgetTest.php

@@ -13,8 +13,11 @@
 namespace Contao\CoreBundle\Tests\Contao;
 
 use Contao\Input;
+use Contao\System;
 use Contao\Widget;
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\DependencyInjection\ContainerBuilder;
+use Symfony\Component\HttpFoundation\RequestStack;
 
 /**
  * @group contao3
@@ -44,6 +47,11 @@ public function setUp(): void
         parent::setUp();
 
         \define('TL_MODE', 'FE');
+
+        $container = new ContainerBuilder();
+        $container->set('request_stack', new RequestStack());
+
+        System::setContainer($container);
     }
 
     /**