AttributeError: 'Breadcrumbs' object has no attribute 'root_node'

Package version (if known): 0.5.1

## Describe the bug
I'm wondering if something has changed within Flask-Menu that Flask-Breadcrumbs needs to take into account.

## Steps to Reproduce
1. Install latest releases of Flask-Breadcrumbs and Flask-Menu (0.5.1 and 1.0.0 respectively)
2. Run the [Simple Example](https://flask-breadcrumbs.readthedocs.io/en/latest/#simple-example)
3. Visit the [app](http://127.0.0.1:5000)
4. Profit...er, error.

## Expected behavior
`TypeError: The view function for 'index' did not return a valid response. The function either returned None or ended without a return statement.`
Ideally there would be no error, but I think index() not returning anything is a (different) bug in the example code.

## Screenshots (if applicable)

## Additional context
Downgrading flask-menu from 1.0.0 to 0.7.2 makes the problem go away. This leads me to believe current Flask-Breadcrumbs is incompatible with current Flask-Menu.