Crash when attenuation is enabled for point clouds

### What happened?

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

## Description

Issue 1 in the forum post

Cesium crashes when we enable attenuation for point clouds.

This seems to be about Camera.main access [here](https://github.com/CesiumGS/cesium-unity/blob/534a41a0137097353eda7c8555307893aff6025c/Source/Runtime/CesiumPointCloudRenderer.cs#L156).

Camera.main is null for us. We use CesiumCameraManager.additionalCameras functionality to define the camera to be used by Cesium.

As a quick test, I used the following function for camera access, and it seems to work:

```
private Camera GetCamera()
{
CesiumCameraManager camManager = GetComponentInParent();
if (camManager == null || camManager.useMainCamera)
return Camera.main;
return camManager.additionalCameras[0];
}

```

### Environment

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

### Reproduction steps

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### Supporting evidence

_No response_