Scroller accessing un-mounted node

[bcomnes]

https://sentry.io/hypermodules/hyperamp/issues/403261476/

TypeError: Cannot match against 'undefined' or 'null'.
  at TrackView.get bottomVisibleRowIndex [as bottomVisibleRowIndex] (/Applications/Hyperamp.app/Contents/Resources/app.asar/renderer/dist/bundle.js:1326:34)
  at TrackView.get midIndexOffset [as midIndexOffset] (/Applications/Hyperamp.app/Contents/Resources/app.asar/renderer/dist/bundle.js:1330:32)
  at window.requestAnimationFrame (/Applications/Hyperamp.app/Contents/Resources/app.asar/renderer/dist/bundle.js:1344:48)

Probably just have to set a default value for https://github.com/hypermodules/hyperamp/blob/eb93a402efc3370982ef6916667ffc197a1091e4/renderer/elements/playlist/track-view.js#L51-L54