The BuildDemo class has been removed but it still used from a route

The \Drupal\form_example\Form\BuildDemo class has been removed in #3593852: Remove the \Drupal\form_example\Form\BuildDemo class, but a route still uses it.

form_example.build_demo:
  path: '/examples/form-example/build-demo'
  defaults:
    _form: '\Drupal\form_example\Form\BuildDemo'
    _title: 'Build form demo'
  requirements:
    _permission: 'access content'

That route needs to be removed.