Models broken when Globe is disabled

[mramato]

The below code snippet fails because globe is false, removing the line fixes the example. This is a regression but I'm not sure which version is stopped working in. (Note you have to remove the line, not set it to true because of the way Viewer options work.)

var viewer = new Cesium.Viewer('cesiumContainer', {
    globe: false
});


var position = Cesium.Cartesian3.fromDegrees(-123.0744619, 44.0503706, 5000);
var heading = Cesium.Math.toRadians(135);
var pitch = 0;
var roll = 0;
var hpr = new Cesium.HeadingPitchRoll(heading, pitch, roll);
var orientation = Cesium.Transforms.headingPitchRollQuaternion(position, hpr);

var entity = viewer.entities.add({
    position : position,
    orientation : orientation,
    model : {
        uri : '../../SampleData/models/CesiumAir/Cesium_Air.glb',
        minimumPixelSize : 128,
        maximumScale : 20000
    }
});
viewer.trackedEntity = entity;

Here's the callstack

TypeError: Cannot read property 'terrainProviderChanged' of undefined
    at Scene.get (http://cesiumjs.org/Cesium/Source/Scene/Scene.js:919:34)
    at new Model (http://cesiumjs.org/Cesium/Source/Scene/Model.js:532:18)
    at Function.Model.fromGltf (http://cesiumjs.org/Cesium/Source/Scene/Model.js:1203:21)
    at ModelVisualizer.update (http://cesiumjs.org/Cesium/Source/DataSources/ModelVisualizer.js:121:31)
    at DataSourceDisplay.update (http://cesiumjs.org/Cesium/Source/DataSources/DataSourceDisplay.js:262:37)
    at Viewer._onTick (http://cesiumjs.org/Cesium/Source/Widgets/Viewer/Viewer.js:1572:49)
    at Event.raiseEvent (http://cesiumjs.org/Cesium/Source/Core/Event.js:142:30)
    at Clock.tick (http://cesiumjs.org/Cesium/Source/Core/Clock.js:308:21)
    at CesiumWidget.render (http://cesiumjs.org/Cesium/Source/Widgets/CesiumWidget/CesiumWidget.js:685:43)
    at render (http://cesiumjs.org/Cesium/Source/Widgets/CesiumWidget/CesiumWidget.js:71:32)

This is probably multiple trivial bugs. Scene should not access the globe unless it's defined and Model should not subscribe to the terrainChanged event if it's undefined.

[mramato]

@ggetz this is probably a quickie if you have some time.