It seems like $container->get('doctrin...al.default_connection') can also be of type null; however, parameter $connection of AppBundle\ShowUnusedMySQ...les\Task::__construct() does only seem to accept Doctrine\DBAL\Connection, maybe add an additional type check?
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Ignorable by Annotation
)
If this is a false-positive, you can also ignore this issue in your code via the ignore-type annotation