1. A concise explanation of the problem you're experiencing.
Periodically my Cesium instance will crash with the following exception:
TypeError: Cannot read property 'update' of undefined
at StaticGeometryPerMaterialBatch.update (https://***/CesiumUnminified/Cesium.js:127895:33)
In StaticGeometryPerMaterialBatch.prototype.update it appears that an array length is determined early, but then code is executed that may destroy elements of the array. That initial length is then used on a subsequent call, ostensibly trying to use indexes of elements that no longer exists.
Skipping any undefined items seems to keep the instance going, but this is so deep in Cesium logic that I have no idea of the implications of doing that.
4. The Cesium version you're using, your operating system and browser.
Cesium 1.36, Chrome 60.0.3112.101, Windows 7 Pro