core/modules/media/src/MediaHandlerBase.php | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/core/modules/media/src/MediaHandlerBase.php b/core/modules/media/src/MediaHandlerBase.php
index a1f9dcb..a52a1f1 100644
--- a/core/modules/media/src/MediaHandlerBase.php
+++ b/core/modules/media/src/MediaHandlerBase.php
@@ -124,6 +124,11 @@ public function defaultConfiguration() {
    * {@inheritdoc}
    */
   public function shouldUpdateThumbnail(MediaInterface $media, $is_new = FALSE) {
+    // @todo This should never be possible, fixing the auto_create_source_field stuff will likely also remove the need for this.
+    if (empty($this->configuration['source_field'])) {
+      return TRUE;
+    }
+
     $source_field_name = $this->getSourceField($media->bundle->entity)->getName();
 
     // Update thumbnail if we don't have a thumbnail yet, for new items,