WebGL crash when point clouds are enabled

### What happened?

**Forum Post:** https://community.cesium.com/t/camera-main-access-at-cesiumforpointcloudrenderer/46110
**Category:** Cesium for Unity
**Reported by:** @myavuzselim

## Description

Issue 3 in the forum post

> On WebGL, we get a crash once we enable point clouds:
>
> `Invoking error handler due to TypeError: Cannot read properties of undefined (reading ‘explicitUniformLocations’), at WebGL.framework.js.gz`
>
> It is mentioned in [that blog post](https://unity.com/blog/cesium-for-unity-3d-geospatial-web) that point clouds are not supported for WebGL. However, I think it should not crash even if it does not render anything.
>
> I was hoping that enabling attenuation affects these bugs in a positive way, but I don’t know.

### Environment

Cesium for Unity version:
Unity Editor Version:
Operating System:

### Reproduction steps

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### Supporting evidence

_No response_

Start by reviewing the linked forum post and reproducing a WebGL build with point clouds enabled. Inspect the reported TypeError involving WebGL.framework.js.gz and compare behavior with point clouds disabled. Done means enabling point clouds no longer crashes, even if point clouds remain unsupported or do not render.

Written by the indexing model from the issue text.