Fix crash when morphing from 2D to 3D is cancelled.

[bagnell]

Fixes #6929.

A better way to reproduce this consistently is to:

1. Switch to 2D
2. Zoom in a bit.
3. Switch to 3D.
4. Cancel the morph by clicking the canvas.

[cesium-concierge]

Thanks for the pull request @bagnell!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[hpinkos]

Looks good, thanks @bagnell!