TypeError: Cannot access offset of type string on string in image_entity_presave() line 353 web/core/modules/image/image.module

Circling back to this in case anyone else has the same issue.

I had one field in our Drupal 7 site that had a default image, causing this problem. I simply removed the default image. Remove the default image via the D7 frontend or modify the value in the database within the field_config_instance table.

I replaced:
s:13:"default_image";s:5:"53425";

with:
s:13:"default_image";i:0;

While running the migration (import) you can patch web/core/modules/image/image.module with

  if ($entity->isSyncing()) {
    return;
  }

  // Added temporary fix for #3306941.
  if (!is_array($default_image) || !isset($default_image['uuid'])) {
    return;
  }
  // End fix #3306941.

  $uuid = $default_image['uuid'];