Fix UserBook save

api_koreader/models.py

@@ -47,7 +47,7 @@ def post_save_document(sender, instance: SyncDocumentEntry, **kwargs):
 @receiver(post_save, sender=UserBook)
 def post_save_userbook(sender, instance: UserBook, **kwargs):
     # Stop recursion
-    if instance.last_progress_device is not None and instance.last_progress_device.startswith("KoReader"):
+    if instance.last_progress_device is None or instance.last_progress_device.startswith("KoReader"):
         return
     # Find the corresponding SyncDocumentEntry
     sync_doc_q: QuerySet[SyncDocumentEntry] = SyncDocumentEntry.objects.filter(user=instance.user, key__iexact=instance.book.partial_md5)