minor

app.py

@@ -69,6 +69,8 @@ def load_header(header_file):
     return content
 
 def preprocess_img(img_path, max_size=1024):
+    if img_path is None:
+        return None
     img = Image.open(img_path)
     if max(img.size) > max_size:
         img.thumbnail((max_size, max_size))