[BUG] WSOD when entity passed to AbstractGenerateDerivativeMediaFile::generateData is not a Media

[seth-shaw-asu]

What steps does it take to reproduce the issue?

• Add a Media derivative action that gets triggered in a Node context. (I don't have a sandbox example as our derivative reaction is a local development)
• Be greeted with a WSOD.

See details @ asulibraries/islandora-repo#550

TL;DR: AbstractGenerateDerivativeMediaFile::generateData returns nothing if triggered on a Node, but this Null is then passed to the StompHeaderEvent which then chokes and WSODs. A properly configured site shouldn't have this scenario occur, but we should be more forgiving when it does happen by throwing an exception which can be caught and reported more gracefully.

Which version of Islandora are you using?

2.x

Any related open or closed issues to this bug report?

No.

[seth-shaw-asu]

Resolved with Islandora/islandora@62fbc6d