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 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/symfony/symfony/src/Symfony/Bundle/FrameworkBundle  /Test/KernelTestCase.php:219

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.