Gets the first feature in session of type feature_type, provided that it is not disabled for msg.
As with get_feature, this should only be used for features where
feature_type is only expected to match a single feature. In particular, if there are two matching features, and the first is
disabled on msg, and the second is not, then this will return null, not the second feature.
feature is owned by session.
| msg | a Message |
| feature_type | the Type of the feature to get |
a SessionFeature, or null. The |