"Error: Animation with null timeline is not supported" is incorrectly thrown when creating a new Animation using web-animations-next.min.js version 2.3.2 on Safari 14.0.2.

let effect = new KeyframeEffect(box, { opacity: 0 }, 300);
let animation = new Animation(effect);
// -> Error: Animation with null timeline is not supported

As per the spec, timeline is:

An optional value which, if present, specifies the timeline with which to associate the newly-created animation. If missing, the default document timeline of the Document associated with the Window that is the current global object is used.