Crash when setting model's color when allowPicking is false

[lilleyse]

There is a crash when a Model constructed with allowPicking : false is assigned a color.

The problematic line is here: https://github.com/AnalyticalGraphicsInc/cesium/blob/master/Source/Scene/Model.js#L4581

It should check that pickCommand is defined first, since it won't be defined if allowPicking is false.

@likangning93 could you check this one out? It's semi-related to the clipping planes updates.

Sandcastle example:
https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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