Updating 5.4.4 => 5.5.4: Problems with integration tests

We’re using the KernelTestCase integration test base class and since our upgrade to 5.5.4 there’s an error calling the container in the Kernel::shutdown method:

// cleanup runtime cache, doctrine, monolog ... to free some memory and avoid locking issues
$this->container->get(\Pimcore\Helper\LongRunningHelper::class)->cleanUp();

This error comes up in the setUp method of our tests when Pimcore tries to “restart” the kernel for testing.

Error: Call to a member function get() on null
/var/www/pimcore/vendor/pimcore/pimcore/lib/Kernel.php:175
/var/www/pimcore/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle  /Test/KernelTestCase.php:219
/var/www/pimcore/vendor/symfony/symfony/src/Symfony/Bundle/FrameworkBundle/Test/KernelTestCase.php:165

Do we have to use another implementation or does anyone knows what we’re doing wrong?
We have over 450 integration tests and till 5.4.4 it worked.