Summary
A few small changes.
Changes
- Checks Scene Descriptor for null spawns object before trying to get length, fixing issue when adding a Scene Descriptor to your world manually.
- Prompts user to save their scene before loading a Sample Scene, fixing [this Canny issue](https://feedback.vrchat.com/sdk-bug-reports/p/switching-to-the-example-scene-deletes-unsaved-changes.