Fix terrain clipping planes sandcastle example

[hpinkos]

Fixes #6366

Users should never scene.primitives.removeAll when they're using the entity layer.

I'm worried this might be a common problem though. The reason this crash was introduced is because DataSources have their own PrimitiveCollection now. Calling scene.primitives.removeAll removes and destroys the data source's primitive collection from primitives. Then later on StaticOutlineGeometryBatch tried to remove a primitive from the destroyed PrimitiveCollection on the DataSource and that's where the error came from.

I don't know a way around this. Do you think we should add a private PrimitiveCollection on scene that all the entitiy primitives get dumped into? That way the end users wouldn't accidentally delete them out from underneath the entity layer.

[cesium-concierge]

@hpinkos, thanks for the pull request! Maintainers, we have a signed CLA from @hpinkos, so you can review this at any time.

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[mramato]

This has been a known problem forever, so don't stress too much. Basically, no one should ever call scene.primitives.removeAll.

[hpinkos]

Okie dokie. I was just concerned because before users would just be like 'why can't I see my polygon' and now they'll be like 'why is my whole application crashing?'

[mramato]

There were plenty of other scenarios where this would lead to a crash (for example, calling removeAll while entities were still in the scene crashes in most cases).