It seems like $dbName can also be of type null; however, parameter $database of Bdf\Prime\Schema\Manager...rface::createDatabase() does only seem to accept string, 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
$this->io->line('Database <comment>%s</comment> has been <info>created</info> for connection <comment>%s</comment>.', $dbName, $connection->getName());