Fix index issue for contao5

[Defcon0]

Hello folks,

after installing the bundle to contao 5.6, I get the following error after going to the themes in the backend:

TypeError:
Contao\CoreBundle\DataContainer\DataContainerOperationsBuilder::generateOperation(): Argument #1 ($name) must be of type string, int given, called in /var/www/html/vendor/contao/core-bundle/src/DataContainer/DataContainerOperationsBuilder.php on line 91

  at vendor/contao/core-bundle/src/DataContainer/DataContainerOperationsBuilder.php:200
  at Contao\CoreBundle\DataContainer\DataContainerOperationsBuilder->generateOperation()
     (vendor/contao/core-bundle/src/DataContainer/DataContainerOperationsBuilder.php:91)
  at Contao\CoreBundle\DataContainer\DataContainerOperationsBuilder->initializeWithButtons()
     (vendor/contao/core-bundle/contao/classes/DataContainer.php:887)
  at Contao\DataContainer->generateButtons()
     (vendor/contao/core-bundle/contao/drivers/DC_Table.php:4990)
  at Contao\DC_Table->listView()
     (vendor/contao/core-bundle/contao/drivers/DC_Table.php:465)
  at Contao\DC_Table->showAll()
     (vendor/contao/core-bundle/contao/classes/Backend.php:465)
  at Contao\Backend->getBackendModule()
     (vendor/contao/core-bundle/contao/controllers/BackendMain.php:144)
  at Contao\BackendMain->run()
     (vendor/contao/core-bundle/src/Controller/BackendController.php:44)
  at Contao\CoreBundle\Controller\BackendController->mainAction()
     (vendor/symfony/http-kernel/HttpKernel.php:181)
  at Symfony\Component\HttpKernel\HttpKernel->handleRaw()
     (vendor/symfony/http-kernel/HttpKernel.php:76)
  at Symfony\Component\HttpKernel\HttpKernel->handle()
     (vendor/symfony/http-kernel/Kernel.php:197)
  at Symfony\Component\HttpKernel\Kernel->handle()
     (public/index.php:42)
  at require('/var/www/html/public/index.php')
     (public/app.php:13)    

Problem is that the index is an int which isn't supported anymore in contao 5 and php 8+.

The PR fixes that.

Bye Defcon0