Error when no media item found

Hi and thank you for catching this. Maybe there was a referenced entity that was removed? I was able to duplicate the issue by zero-ing out the "target_id" so that it would not load the media or thumbnail correctly:

 // Load the media for the thumbnail, if it's media reference.
else {
+   $target_id = 0;            
    $thumbnail_media = $this->entityTypeManager->getStorage('media')->load($target_id);

Regarding your patch, I feel that the thumbnail_file_id needed to be set either way, even if to false, but further down needed some checks.

I have pushed a fix to the 1.x-dev branch.

Testing Instructions
- Use the MVM formatter with "Custom Thumbnail Media Reference" display
- Use the code block above to manually alter the target id
- Verify the page still loads, but the MVM item would not display since the thumbnail did not exist

Tested on:
Drupal 10.3.6
Drupal 11.0.5

RTBC +1

pushing this to new release

Thanks @joshahubbers!