Never assume media entities passed to the MediaType plugin have their required or "source" fields set.

Problem/Motivation

Based on some comments in #2747173: Fatal error: Call to a member function downloadThumbnail() on a non-object and the open issue #2781833: "Multi step selection display" somehow causes an empty media entity to enter MediaType plugins., it looks like some interaction between inline entity form, entity browser and the multi-step selection display cause a media entity to be saved without any of it's fields set. I've stepped through this code and am struggling to understand the problem.

The other MediaType plugins are more resilient to this, perhaps we should do the same?

Proposed resolution

Patch and see what the impact is.

Remaining tasks

Patch.

User interface changes

None.

API changes

None.

Data model changes

None.