PolylineGeometryUpdater crash when deleting and re-creating an object position

[shunter]

Sandcastle example

(load then click the Test button)

This example creates a polyline via CZML, then processes a second document which deletes one of the endpoint positions, then immediately recreates it.

This crashes:

Sandcastle-client.js:20 TypeError: Cannot read property 'getValueInReferenceFrame' of undefined
    at ReferenceProperty.getValueInReferenceFrame (Cesium.js:128476)
    at PositionPropertyArray.getValueInReferenceFrame (Cesium.js:128013)
    at PositionPropertyArray.getValue (Cesium.js:127979)
    at PolylineGeometryUpdater._onEntityPropertyChanged (Cesium.js:155733)
    at Event.raiseEvent (Cesium.js:12972)
    at Entity.<anonymous> (Cesium.js:80710)
    at Event.raiseEvent (Cesium.js:12972)
    at PolylineGraphics.<anonymous> (Cesium.js:80710)
    at Event.raiseEvent (Cesium.js:12972)
    at PositionPropertyArray._raiseDefinitionChanged (Cesium.js:128063)

This is because PolylineGeometryUpdater.prototype._onEntityPropertyChanged responds to the property being deleted by immediately trying to get the value, and ReferenceProperty crashes when it is asked to get the value.

@mramato do you remember why we made ReferenceProperty crash rather than return undefined? It's been so long that I don't know if there was a reason or not.

[mramato]

The only explanation I have is very early on, we assumed properties were never undefined and that we wanted to "fail fast" instead of letting bad/unexpected data in. Obviously that's changed quite a bit (For the better).