This PR was submitted for the 2.8 branch but it was merged into the 2.7 branch instead (closes#26183).Discussion----------[DI] Add null check for removeChild| Q             | A| ------------- | ---| Branch?       | 2.8| Bug fix?      | yes| New feature?  | no| BC breaks?    | no| Deprecations? | no| Tests pass?   | yes| Fixed tickets || License       | MIT| Doc PR        |I'm using Symfony 2.8 and PHP Fatal error occurs very rarely when I do assetic:dump / cache:warmup as below.(also on Symfony 2.7)`PHP Fatal error:  Call to a member function removeChild() on null in .../symfony/symfony/src/Symfony/Component/DependencyInjection/Loader/XmlFileLoader.php on line 350`It does occur very rarely. Also, after several retries, it works fine.The xml file which triggers error is as below(both version) and I didn't modify anything.https://github.com/symfony/swiftmailer-bundle/blob/v2.3.8/Resources/config/swiftmailer.xmlhttps://github.com/symfony/swiftmailer-bundle/blob/v2.6.7/Resources/config/swiftmailer.xmlOnce more, error occurs so randomly. So, I can't be sure in what situation it occurs. But this modification does fix the problem and no side effect.Commits-------a5f05a0 [DI] Add null check for removeChild