Update app.py

Co-authored-by: coderabbitai[bot] <136622811+coderabbitai[bot]@users.noreply.github.com>

Author: guibranco

app.py

@@ -55,8 +55,7 @@ def get_style_fields(style):
       return {}
 
     # Handles case sensitivity
-    style = style.lower()
-
+    style = style.lower() if style is not None else "default"
     style_templates = {
         "default": {
             "title": "",