Dynamic entities expect to live forever?

This [came up in this forum thread](https://groups.google.com/d/msg/cesium-dev/2Nzjsy9-YFQ/d63m46j2BAAJ).

This is an entity with an ellipsoid with a dynamic radius. Trying to change it to an ellipse at run-time crashes because the geometry updater expects the entity to always be defined.

[Sandcastle link](https://cesiumjs.org/Cesium/Apps/Sandcastle/index.html#c=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).

This works fine if the ellipsoid is not dynamic. This seems to be a problem with all dynamic entities. Originally, when I asked why CesiumJS was designed to have one abstract "entity" with properties for each geometry type, instead of just creating `new PolygonEntity()`, the reason was that an Entity can be one shape, multiple shapes at the same time, or different shapes across time.

So I think it makes sense to consider this a bug, that you can't get rid of dynamic entities by setting them to undefined.

Start by reproducing the crash with the linked Sandcastle example: use an entity with a dynamic ellipsoid radius, then change the geometry to undefined at runtime. Trace the geometry updater involved in that transition and verify that dynamic entities can be removed without a crash, including when their geometry changes over time.

Written by the indexing model from the issue text.