Crash when switching scene modes during flight

Found during release testing, but confirmed to happen as far back as 1.25 so I don't think it needs to hold it up. We should fix it for 1.28 however.

1. Run Cesium Viewer and switch to 2D
2. Zoom in anywhere.
3. Press the home button and switch to 3D during the flight.
4. Click anywhere to immediately finish the morph.
5. Crash

```
TypeError: Unable to get property 'height' of undefined or null reference
TypeError: Unable to get property 'height' of undefined or null reference
at Camera.prototype._updateCameraChanged (http://localhost:8080/Source/Scene/Camera.js:333:9)
at Scene.prototype.initializeFrame (http://localhost:8080/Source/Scene/Scene.js:2338:9)
at CesiumWidget.prototype.render (http://localhost:8080/Source/Widgets/CesiumWidget/CesiumWidget.js:680:13)
at render (http://localhost:8080/Source/Widgets/CesiumWidget/CesiumWidget.js:72:25)
```

Reproduce the crash in Cesium Viewer by following the listed 2D-to-3D flight steps. Start with Camera.js at _updateCameraChanged, then inspect the calls from Scene.js and CesiumWidget.js shown in the stack trace. Done means switching modes during flight and finishing the morph no longer throws the reported TypeError.

Written by the indexing model from the issue text.