Add back end module check

[fritzmg]

If a user does not have access to the lead back end module in their group settings, the following error will currently occur:

ErrorException:
Warning: Undefined array key "label"

  at vendor\contao\core-bundle\src\EventListener\Menu\BackendMenuListener.php:75
  at Contao\CoreBundle\EventListener\Menu\BackendMenuListener->buildMainMenu(object(MenuEvent), object(BackendUser))
     (vendor\contao\core-bundle\src\EventListener\Menu\BackendMenuListener.php:56)
  at Contao\CoreBundle\EventListener\Menu\BackendMenuListener->__invoke(object(MenuEvent), 'contao.backend_menu_build', object(TraceableEventDispatcher))
     (vendor\symfony\event-dispatcher\Debug\WrappedListener.php:118)
  at Symfony\Component\EventDispatcher\Debug\WrappedListener->__invoke(object(MenuEvent), 'contao.backend_menu_build', object(TraceableEventDispatcher))
     (vendor\symfony\event-dispatcher\EventDispatcher.php:230)
  at Symfony\Component\EventDispatcher\EventDispatcher->callListeners(array(object(WrappedListener), object(WrappedListener), object(WrappedListener), object(WrappedListener)), 'contao.backend_menu_build', object(MenuEvent))
     (vendor\symfony\event-dispatcher\EventDispatcher.php:59)
  at Symfony\Component\EventDispatcher\EventDispatcher->dispatch(object(MenuEvent), 'contao.backend_menu_build')
     (vendor\symfony\event-dispatcher\Debug\TraceableEventDispatcher.php:154)
  at Symfony\Component\EventDispatcher\Debug\TraceableEventDispatcher->dispatch(object(MenuEvent), 'contao.backend_menu_build')
     (vendor\contao\core-bundle\src\Menu\BackendMenuBuilder.php:42)
  at Contao\CoreBundle\Menu\BackendMenuBuilder->buildMainMenu(array())
     (vendor\knplabs\knp-menu\src\Knp\Menu\Provider\LazyProvider.php:39)
  at Knp\Menu\Provider\LazyProvider->get('be_menu', array())
     (vendor\knplabs\knp-menu\src\Knp\Menu\Provider\ChainProvider.php:26)
  at Knp\Menu\Provider\ChainProvider->get('be_menu', array())
     (vendor\knplabs\knp-menu\src\Knp\Menu\Twig\Helper.php:43)
  at Knp\Menu\Twig\Helper->get('be_menu', array())
     (vendor\knplabs\knp-menu\src\Knp\Menu\Twig\Helper.php:142)
  at Knp\Menu\Twig\Helper->castMenu('be_menu')
     (vendor\knplabs\knp-menu\src\Knp\Menu\Twig\Helper.php:75)
  at Knp\Menu\Twig\Helper->render('be_menu', array(), null)
     (vendor\knplabs\knp-menu\src\Knp\Menu\Twig\MenuExtension.php:73)
  at Knp\Menu\Twig\MenuExtension->render('be_menu')
     (var\cache\dev\twig\e2\e2f6004b8a7987f4e4613cc3c7b1d30d.php:62)
  at __TwigTemplate_04d06aa6eece0e811370c7892dbb645c->doDisplay(array('app' => object(AppVariable)), array())
     (vendor\twig\twig\src\Template.php:387)
  at Twig\Template->yield(array('app' => object(AppVariable)), array())
     (vendor\twig\twig\src\Template.php:343)
  at Twig\Template->display(array())
     (vendor\twig\twig\src\Template.php:358)
  at Twig\Template->render(array())
     (vendor\twig\twig\src\TemplateWrapper.php:35)
  at Twig\TemplateWrapper->render(array())
     (vendor\twig\twig\src\Environment.php:320)
  at Twig\Environment->render('@ContaoCore/Backend/be_menu.html.twig')
     (vendor\contao\core-bundle\src\Resources\contao\controllers\BackendMain.php:271)
  …

This is because Contao will not pre-fill the $modules array with the lead module and its label if access to the lead back end module is not granted. But the onGestUserNavigation listener will still add the leads the user has access to, thus the rest of the data for the back end navigation (like the label etc.) will be missing.