SWIFTMAILER_TRANSPORT_SMTP missing when enabling the module

I couldn't reproduce the error during installation of the module. I then tested a custom cron where I tried to call the constant and I did get the WSOD. Not sure why it didn't crash out during installation.

Anyway, here's a patch for this

Thanks you so much sidharth_soman,
The module is well enabling now.

There is already a test for this on line 46.
\Drupal::moduleHandler()->moduleExists('swiftmailer'))

How can the module exist but SWIFTMAILER_TRANSPORT_SMTP is not defined