Classification extension not working with classes

[remicres]

How to reproduce

from pystac.extensions.classification import ClassificationExtension, Classification

item = pystac.Item(...)
CLASSES = [{"value": 2,  "name": "cloud", "description": "Clouds", "color_hint": "B8D0EC"}, ...]

classif_ext = ClassificationExtension.ext(item.assets["asset_key"], add_if_missing=True)
classes = [Classification.create(**cls) for cls in CLASSES]
classif_ext.apply(classes=classes)
print(classif_ext.classes)

What we see is None.
However we should see {'classification:classes': [{'value': 2, 'name': 'cloud', 'description': 'Clouds', 'color_hint': 'B8D0EC'}, ...]}

[gadomski]

Closed by #1484