checking if copyDepthCommand is defined before destroying its shaderProgram
checking if copyDepthCommand is defined before destroying its shaderProgram

likangning93/cesium,CesiumGS/cesium,CesiumGS/cesium,likangning93/cesium,likangning93/cesium,progsung/cesium,AnalyticalGraphicsInc/cesium,CesiumGS/cesium,likangning93/cesium,CesiumGS/cesium,progsung/cesium,CesiumGS/cesium,AnalyticalGraphicsInc/cesium,likangning93/cesium