Fix crash during zoom for unloaded tileset

[j9liu]

Description

This PR fixes a crash that occured when zooming to a tileset with Update in Editor disabled. This crash resulted from a lack of nullptr guard in ACesium3DTileset::OnFocusEditorViewportOnThis.

Issue number or link

N/A

Author checklist

• I have submitted a Contributor License Agreement (only needed once).
• I have done a full self-review of my code.
• I have updated CHANGES.md with a short summary of my change (for user-facing changes).
• [ ] I have added or updated unit tests to ensure consistent code coverage as necessary.
  • I did try to add a unit test for this, however the automated test suite runs in Play-in-Editor mode, which ignores the "Update in Editor" setting entirely. I'm not sure how to simulate it running in-Editor.
• [ ] I have updated the documentation as necessary.

Testing plan

1. Open 01_CesiumWorld in the Samples project.
2. On Cesium World Terrain, switch the tileset source to From Url and disable Update in Editor.
3. Click Refresh Tileset.
4. Double-click on Cesium World Terrain in the Outliner. Without this PR, the crash happens here. With this PR, the Editor should not crash.

Reviewer checklist

Thank you for taking the time to review this PR. By approving a PR you are taking as much responsibility for these changes as the author.

As you review, please go through the checklist below:

• Review and run all parts of the test plan on this branch and verify it matches expectations.
  • If the issue is a bug please make sure you can reproduce the bug in the main branch and then checkout this branch to make sure it actually solved the issue.
• Review the code and make sure you do not have any remaining questions or concerns. You should understand the code change and the chosen approach. If you are not confident or have doubts about the code, please do not hesitate to ask questions.
  • Code should follow the C++ Style Guide
• Review the unit tests and make sure there are no missing tests or edge cases.
• Review documentation changes and updates to CHANGES.md to make sure they accurately cover the work in this PR.
• Verify that the Contributor License Agreement has been submitted, if needed.

[azrogers]

Thanks @j9liu!