Fix WSOD if referenced media gets deleted

[bserem]

What does this change?

• Call Out Box paragraph, found under localgov_subsites_paragraphs uses a media field named localgov_background_image.
• Data from that field is then loaded in
  

  localgov_paragraphs/modules/localgov_subsites_paragraphs/localgov_subsites_paragraphs.module

  Line 147 in f270b7a

  $fid = $paragraph->get('localgov_background_image')->entity->field_media_image[0]->getValue()['target_id'];

• If a media file that is used in Call Out Box gets deleted then the site results in a WSOD and the user can't view or edit the node in order to fix it.

Checking if the field is empty:

localgov_paragraphs/modules/localgov_subsites_paragraphs/localgov_subsites_paragraphs.module

Line 146 in f270b7a

if (!$paragraph->get('localgov_background_image')->isEmpty()) {

is not correct in this case, as we should be checking if the entity actually exists

How to test

1. Add a node with a call out box with a media
2. See the node
3. Delete the media from the media library
4. See the node -> you get a PHP error

How can we measure success?

Deleting a media doesn't break the page

[ekes]

This isn't the first time this section has caused us pain.
#125

Feels like it's brittle and needs tests. There is however nothing and this will fix the specific issue.

[stephen-cox]

This fixes the issue, but we're getting the entity to check if it exists and then again on the next line to get the file ID.

I suggest assigning the entity to a variable when checking for it doesn't need to be fetched for a second time.

E.g.

    if ($file = $paragraph->get('localgov_background_image')->entity) {
      $fid = $file->entity->field_media_image[0]->getValue()['target_id'];

Agree with @ekes that tests for all of this are needed, but we can create another issue for that.

[stephen-cox]

@bserem Happy to approve this if you're able to make the requested code change 🙂

[finnlewis]

@ekes and @stephen-cox still keen to make the change before merging.