According to this code part:

The container may be null if the kernel has been shutdown.

But this simply call $this->kernel->getContainer(); and according to the PHPDoc:

It should never return null, always a ContainerInterface.

This also make PHPStan yelling on web tests:

Calling method get() on possibly null value of type Symfony\Component\DependencyInjection\ContainerInterface|null.

If am I right, the PHPDoc of the Client must be corrected.

If it may return null, I suggest to throw an exception instead. This method should not called if the kernel is not booted AFAIK.

What do you think?