code: private function runCommand(Command $command, InputInterface $input): ?string
      {
          if ($command instanceof ContainerAwareInterface) {
              $command->setContainer($this->container);
          }
  
          $output = new BufferedOutput(OutputInterface::VERBOSITY_NORMAL, true);
          $status = $command->run($input, $output);
  
          if ($status > 0) {
              return $output->fetch();
          }
  
          return null;
      }
docs: Runs a command and returns the error (if any).