Update app.py

app.py

@@ -50,8 +50,8 @@ def generate_qr(url, fill_color=None, back_color=None):
             raise ValueError("URL or data cannot be empty")
         
         # Convert empty strings to None
-        fill_color = fill_color.strip() if fill_color else None
-        back_color = back_color.strip() if back_color else None
+        fill_color = fill_color.strip() if fill_color else 'black'
+        back_color = back_color.strip() if back_color else 'white'
         
         # Generate QR code with optional color parameters
         return get_qr_image(url, fill_color, back_color)