Configuration value for media type is unset

[ctgraham]

After initially providing a value for the media type in the form, revisiting the form to edit the Action leave the media type value blank.

There is an orphaned variable reference which is the root cause here:

xslt_derivative/src/Plugin/Action/XsltDerivative.php

Line 303 in cf8c107

->getQuery()->condition('id', $this->configuration['plaintext_media_type'])->execute();