Error when playing a nested sibling fill forwards group effect

[alancutter]

Repro:

document.timeline.play(
    new SequenceEffect([
      new GroupEffect([new KeyframeEffect(null, [], 100)]),
      new GroupEffect([new KeyframeEffect(null, [], 100)], {fill: 'forwards'}),
    ])
);

Stack trace:

web-animations-next-animation.js:133 Uncaught TypeError: Cannot read property '_setExternalAnimation' of undefined
    at b.Animation._setExternalAnimation (web-animations-next-animation.js:133)
    at b.Animation._setExternalAnimation (web-animations-next-animation.js:133)
    at Object.b.bindAnimationForGroup (group-constructors.js:199)
    at b.Animation._rebuildUnderlyingAnimation (web-animations-next-animation.js:95)
    at new b.Animation (web-animations-next-animation.js:39)
    at b.AnimationTimeline._play (timeline.js:48)
    at b.AnimationTimeline.play (timeline.js:65)
    at <anonymous>:1:19