Multipage App - app has no attribute callback

Dash Python
1

trying to run the “Structuring a Multi-Page App” example in https://dash.plotly.com/urls following the folders structure

- app.py
- index.py
- apps
   |-- __init__.py
   |-- app1.py
   |-- app2.py

but I am getting an error "module ‘src.dash.app’ has no attribute ‘callback’’