instanceIndex Error when removing polylines

1. A concise explanation of the problem you're experiencing.
I'm getting and instanceIndex error when removing polylines. This cause seems to be from this issue, but I'm not quite sure if it is something that can happen due to misuse of the framework's code. I don't see anywhere in our code where we directly call PolylineCollection.update() btw.

2. A minimal code example. If you've found a bug, this helps us reproduce and repair it.

3. Context. Why do you need to do this? We might know a better way to accomplish your goal.
Create and remove polylines without error

4. The Cesium version you're using, your operating system and browser.

Looks like you’ve already figured this out:

Looks like this might be fixed and merged soon, so thanks for hunting this down!