fix: document delete image files check file exist

api/core/indexing_runner.py

@@ -325,7 +325,8 @@ class IndexingRunner:
                             "Delete image_files failed while indexing_estimate, \
                                           image_upload_file_is: {}".format(upload_file_id)
                         )
-                    db.session.delete(image_file)
+                    if image_file:
+                        db.session.delete(image_file)
 
         if doc_form and doc_form == "qa_model":
             return IndexingEstimate(total_segments=total_segments * 20, qa_preview=preview_texts, preview=[])