add warning log to fix crashing bug

src/backend/threads_availchecker.py

@@ -56,6 +56,9 @@ class AvailChecker(QThread):
             rds = transformer.get_data(data).return_data("rds_availability")
 
             book_id = None
+            if not rds or not rds.items:
+                log.warning(f"No RDS data found for link {link}")
+                continue
             for item in rds.items:
                 sign = item.superlocation
                 loc = item.location